xgettext: Avoid side effects of the Vala extractor on the C extractor.

xgettext: Avoid side effects of the Vala extractor on the C extractor.

* gettext-tools/src/xgettext.h (xgettext_record_flag): Add more comments.
* gettext-tools/src/xgettext.c (xgettext_record_flag): Parse an optional
'!BACKEND' suffix. When present, limit the effects of c-format related flags to
the specified flag table(s).
* gettext-tools/src/x-vala.c (init_flag_table_vala): Append '!Vala' to all
xgettext_record_flag arguments.
* gettext-tools/tests/xgettext-c-format-6: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add it.