Fix ShellCheck warnings
Fix ShellCheck warnings

```
In check_ironport_snmp line 9:
if [ ! -z ${DEBUG} ]; then
     ^-- SC2236: Use -n instead of ! -z.
          ^------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 23:
if [ "${1}" = "--help" -o "${#}" = "0" ]; then
                       ^-- SC2166: Prefer [ p ] || [ q ] as [ p -o q ] is not well defined.

In check_ironport_snmp line 68:
    if ! $(which ${CMD} &>/dev/null); then
         ^-------------------------^ SC2091: Remove surrounding $() to avoid executing output.
           ^---^ SC2230: which is non-standard. Use builtin 'command -v' instead.
                 ^----^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 123:
    if [ -z ${WARN_THRESHOLD} ]; then
            ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 128:
    if [ -z ${CRIT_THRESHOLD} ]; then
            ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 133:
    if [ ${CRIT_THRESHOLD} -le ${WARN_THRESHOLD} ]; then
         ^---------------^ SC2086: Double quote to prevent globbing and word splitting.
                               ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 150:
    if [ ${mem} -gt ${CRIT_THRESHOLD} ]; then
         ^----^ SC2086: Double quote to prevent globbing and word splitting.
                    ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 152:
    elif [ ${mem} -gt ${WARN_THRESHOLD} ]; then
           ^----^ SC2086: Double quote to prevent globbing and word splitting.
                      ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 171:
    if [ ${cpu} -gt ${CRIT_THRESHOLD} ]; then
         ^----^ SC2086: Double quote to prevent globbing and word splitting.
                    ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 173:
    elif [ ${cpu} -gt ${WARN_THRESHOLD} ]; then
           ^----^ SC2086: Double quote to prevent globbing and word splitting.
                      ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 192:
    if [ ${diskio} -gt ${CRIT_THRESHOLD} ]; then
         ^-------^ SC2086: Double quote to prevent globbing and word splitting.
                       ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 194:
    elif [ ${diskio} -gt ${WARN_THRESHOLD} ]; then
           ^-------^ SC2086: Double quote to prevent globbing and word splitting.
                         ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 213:
    if [ ${queue} -gt ${CRIT_THRESHOLD} ]; then
         ^------^ SC2086: Double quote to prevent globbing and word splitting.
                      ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 215:
    elif [ ${queue} -gt ${WARN_THRESHOLD} ]; then
           ^------^ SC2086: Double quote to prevent globbing and word splitting.
                        ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 331:
    if [ ${temperature} -gt ${CRIT_THRESHOLD} ]; then
         ^------------^ SC2086: Double quote to prevent globbing and word splitting.
                            ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 333:
    elif [ ${temperature} -gt ${WARN_THRESHOLD} ]; then
           ^------------^ SC2086: Double quote to prevent globbing and word splitting.
                              ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 352:
    if [ ${open_files} -gt ${CRIT_THRESHOLD} ]; then
         ^-----------^ SC2086: Double quote to prevent globbing and word splitting.
                           ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 354:
    elif [ ${open_files} -gt ${WARN_THRESHOLD} ]; then
           ^-----------^ SC2086: Double quote to prevent globbing and word splitting.
                             ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 373:
    if [ ${threads} -gt ${CRIT_THRESHOLD} ]; then
         ^--------^ SC2086: Double quote to prevent globbing and word splitting.
                        ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 375:
    elif [ ${threads} -gt ${WARN_THRESHOLD} ]; then
           ^--------^ SC2086: Double quote to prevent globbing and word splitting.
                          ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 399:
    if [ ${outstanding_dns} -gt ${CRIT_THRESHOLD} -o ${pending_dns} -gt ${CRIT_THRESHOLD} ]; then
         ^----------------^ SC2086: Double quote to prevent globbing and word splitting.
                                ^---------------^ SC2086: Double quote to prevent globbing and word splitting.
                                                  ^-- SC2166: Prefer [ p ] || [ q ] as [ p -o q ] is not well defined.
                                                     ^------------^ SC2086: Double quote to prevent globbing and word splitting.
                                                                        ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 401:
    elif [ ${outstanding_dns} -gt ${WARN_THRESHOLD} -o ${pending_dns} -gt ${WARN_THRESHOLD} ]; then
           ^----------------^ SC2086: Double quote to prevent globbing and word splitting.
                                  ^---------------^ SC2086: Double quote to prevent globbing and word splitting.
                                                    ^-- SC2166: Prefer [ p ] || [ q ] as [ p -o q ] is not well defined.
                                                       ^------------^ SC2086: Double quote to prevent globbing and word splitting.
                                                                          ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 470:
        i=$(($i + 1 ))
             ^-- SC2004: $/${} is unnecessary on arithmetic variables.

In check_ironport_snmp line 525:
        i=$(($i + 1 ))
             ^-- SC2004: $/${} is unnecessary on arithmetic variables.

In check_ironport_snmp line 584:
        i=$(($i + 1 ))
             ^-- SC2004: $/${} is unnecessary on arithmetic variables.

In check_ironport_snmp line 610:
        if [ ${FanValue[${i}]} -gt ${CRIT_THRESHOLD} ]; then
             ^---------------^ SC2086: Double quote to prevent globbing and word splitting.
                                   ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 612:
        elif [ ${FanValue[${i}]} -gt ${WARN_THRESHOLD} ]; then
               ^---------------^ SC2086: Double quote to prevent globbing and word splitting.
                                     ^---------------^ SC2086: Double quote to prevent globbing and word splitting.

In check_ironport_snmp line 635:
        i=$(($i + 1 ))
             ^-- SC2004: $/${} is unnecessary on arithmetic variables.

For more information:
  https://www.shellcheck.net/wiki/SC2091 -- Remove surrounding $() to avoid e...
  https://www.shellcheck.net/wiki/SC2166 -- Prefer [ p ] || [ q ] as [ p -o q...
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ...
```

joschi/check_ironport_snmp