maint: sync autoconf Channels.pm and ChannelsDefs.pm.

maint: sync autoconf Channels.pm and ChannelsDefs.pm.

Patch from https://bugs.gnu.org/67971.

The changes address <https://savannah.gnu.org/support/?110872>,
about m4_warn code/documentation consistency.
It should be impossible to reach report_bad_channel from code in
Automake.

* lib/Automake/Channels.pm (msg): If the channel argument is invalid,
don't crash; report the mistake and use the `syntax' channel.
(report_bad_channel): New function for reporting invalid channels.

* lib/Automake/ChannelDefs.pm (usage): Clarify that the list of
warning categories is exhaustive, and that ``all'', ``none'',
``no-CATEGORY'', and ``error'' are not warning categories.