Fixed #21488 -- Multiple locales treatment in i18n commands.

Removed multiple locales separated by commas variation (that wasn't
working as documented) in favor of simply allowing use of the
``--locale``/``-l`` options more than once for ``makemessages`` and
``compilemessages``.

Thanks Romain Beylerian for the report and Claude, Simon for their help.

8750296918072c97a51f6809d19828ce3f1b8d40 from stable/1.6.x.
fixes #21412 -- messages framework add_message should check request arg

problem were users calling messages.debug/info/* with a wrong argument
and getting the error "You cannot add messages without installing
MessageMiddleware"