libgettextpo: New function po_message_get_format.

libgettextpo: New function po_message_get_format.

Reported by Alexander Potashev <aspotashev@gmail.com> in
<https://savannah.gnu.org/bugs/?67485>.

* gettext-tools/libgettextpo/gettext-po.in.h (po_message_get_format): New
declaration.
* gettext-tools/libgettextpo/gettext-po.c (po_message_get_format): New function.
* gettext-tools/tests/gettextpo-1-prg.c (main): Test the return value of
po_message_get_format. Add a test case of po_message_set_format with value -1.
* gettext-tools/doc/gettext.texi (po_message_t API): Document
po_message_get_format.
* NEWS: Mention the change.