xgettext: Avoid error 'present charset "CHARSET" is not a portable encoding name...

xgettext: Avoid error 'present charset "CHARSET" is not a portable encoding name' when possible.

Reported by Arsen Arsenović <arsen@aarsen.me>
at <https://savannah.gnu.org/bugs/?64490>.

This changes the behaviour back to the one from gettext 0.21.1, where this
xgettext invocation produced just a warning.

* gettext-tools/src/msgl-iconv.c (iconv_message_list_internal): Don't give an
error when a PO file has a header entry with charset=CHARSET and is entirely
ASCII.
* gettext-tools/src/msgl-cat.c (catenate_msgdomain_list): Likewise.
* gettext-tools/tests/xgettext-combine-1: New file.
* gettext-tools/tests/xgettext-combine-2: New file.
* gettext-tools/tests/xgettext-combine-3: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add them.