Refactor: libcrmcommon: NULL checks in pcmk__format_option_metadata()

NULL-check type and shortdesc, both of which the standard requires.

Other minor changes:

• Improve resilience by allowing shortdesc to default to longdesc (instead of only vice-versa).
• Use shorter alias variable names for readability.
• Output the default value only if one is defined.

Ref T152

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>