Move the checks for nmc_arg_is_help to the beginning of the
checks for command matches.

Up to now, no command begins with 'h', so this has no behavioral
change whatsoever. But imagine a command that begins with 'h'
(for example `nmcli general hostname`), in that case `nmcli general h`
should still show the help, as users might be accustomed to this
abbreviation.

Signed-off-by: Thomas Haller <thaller@redhat.com>