ZOOKEEPER-3599: cli.c: Resuscitate "old-style" argument parsing
ZOOKEEPER-3599: cli.c: Resuscitate "old-style" argument parsing

ZOOKEEPER-2122 added SSL support to the C client and to the `cli_st`/`mt` tools.  It introduced (much-welcome!) GNU-style `getopt_long` argument parsing, but did not try to preserve backwards compatibility.

An earlier version of this https://github.com/apache/zookeeper/pull/1131 introduced (optional) POSIX-style `getopt` argument parsing, and was consequently largely obsoleted by the SSL update.  It did, however, support "old-style" arguments to avoid [breaking workflows](https://xkcd.com/1172/).

This patch salvages that feature while preserving the `getopt_long` goodness.  It is "legacy-only"; adding new positional arguments is not supported—as discussed in this thread: https://github.com/apache/zookeeper/pull/1131#pullrequestreview-320870245.

Author: Damien Diederen <dd@crosstwine.com>

Reviewers: Enrico Olivelli <eolivelli@apache.org>, Norbert Kalmar <nkalmar@apache.org>

Closes #1131 from ztzg/ZOOKEEPER-3599-use-getopt-in-cli-c

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