sc_error_message_uppercase: allow fully uppercased words

sc_error_message_uppercase: allow fully uppercased words

If an error message starts with an entire fully uppercased word,
that’s probably a proper noun and it should stay that way.  For
instance, autoreconf has an error message that starts with
"AM_GNU_GETTEXT is used, but ..."; AM_GNU_GETTEXT is the name of an
Automake macro, it needs to stay uppercased.

This subsumes the existing exception for the words FATAL and WARNING.
While I was in there I generalized the exception for PRIuMAX to cover
all of the inttypes.h PRI* and SCN* macros.

This patch has been submitted to Gnulib; until it is merged there,
anyone running ‘make fetch’ should take care not to drop this change.

* maint.mk (sc_error_message_uppercase): Allow error messages that
  begin with any fully uppercased word, or with any of the inttypes.h
  PRI[dioux]\w+ or SCN[dioux]\w+ macros.