xgettext: Check msgid and msgid_plural for compatibility as format strings.

xgettext: Check msgid and msgid_plural for compatibility as format strings.

* gettext-tools/src/if-error.h: Include <stdarg.h>.
(if_verror): New declaration.
* gettext-tools/src/if-error.c (if_verror): New function, extracted from
if_error.
(if_error): Invoke it.
* gettext-tools/src/xg-check.h (xgettext_check_message_list): Renamed from
syntax_check_message_list.
* gettext-tools/src/xg-check.c: Include format.h, if-error.h.
(struct formatstring_error_logger_locals): New type.
(formatstring_error_logger, format_check_message): New functions.
(xgettext_check_message_list): Renamed from syntax_check_message_list. Invoke
also format_check_message.
* gettext-tools/src/xgettext.c (main): Invoke xgettext_check_message_list
instead of syntax_check_message_list.
* gettext-tools/tests/xgettext-19: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add it.
* NEWS: Mention the change.