rev-parse --parseopt: option argument name hints

Built-in commands can specify names for option arguments when usage text
is generated for a command.  sh based commands should be able to do the
same.

Option argument name hint is any text that comes after [*=?!] after the
argument name up to the first whitespace.  Underscores are replaced with
whitespace.  It is unlikely that an underscore would be useful in the
hint text.

Signed-off-by: Ilya Bobyr <ilya.bobyr@gmail.com>