Re-implement CPU metrics collection through sysctls.
Re-implement CPU metrics collection through sysctls.

This removes the requirement to run `node_exporter` as root or with read
access to `/dev/kmem` in order to get CPU usage statistics.

Once FreeBSD adds a macro for the `kern.cp_times` sysctl, the
`setupSysctlMIBs()` function should be replaced by usage of the macro.

chosenken/node_exporter,ema/node_exporter,knweiss/node_exporter,cbstewart9393/node_exporter,chosenken/node_exporter,prometheus/node_exporter,joehandzik/node_exporter,Jean-Daniel/node_exporter,knweiss/node_exporter,siavashs/node_exporter,derekmarcotte/node_exporter,derekmarcotte/node_exporter,knweiss/node_exporter,Jean-Daniel/node_exporter,prometheus/node_exporter,joehandzik/node_exporter,Jean-Daniel/node_exporter,siavashs/node_exporter,siavashs/node_exporter,siavashs/node_exporter,bs-github/node_exporter,ema/node_exporter,ema/node_exporter,bs-github/node_exporter,knweiss/node_exporter,mslocrian/flexswitch_exporter,cbstewart9393/node_exporter,derekmarcotte/node_exporter,Jean-Daniel/node_exporter,prometheus/node_exporter,ema/node_exporter,joehandzik/node_exporter,bs-github/node_exporter,derekmarcotte/node_exporter,joehandzik/node_exporter,mslocrian/flexswitch_exporter,chosenken/node_exporter,bs-github/node_exporter,chosenken/node_exporter