The coloring rule for 'unbind' still checked for the old 'extcmd' and
'externalcmd' menu names. And not coloring binds and unbinds as valid
when followed by something other than a comment was overrestrictive,
because the bind or unbind command would work just fine.
(Nano silently allows trailing garbage on any line without a regex.)
Also, take the opportunity to order the menu names in the bind and
unbind rules in the same way.
This fixes https://savannah.gnu.org/bugs/?61419,
and fixes https://savannah.gnu.org/bugs/?61420.
First bug existed since version 5.0, commit d9106abf.
Second bug existed since version 2.3.3, commit 9c11ba93.
4d9be13316