xgettext: Warn when a message contains an URL or email address.

xgettext: Warn when a message contains an URL or email address.

Reported by Arsen Arsenović <arsen@aarsen.me>
at <https://savannah.gnu.org/bugs/?67000>.

* gettext-tools/src/xg-check.c: Include c-strstr.h.
(SIZEOF): New macro.
(string_has_url, message_has_url, string_has_email, message_has_email,
url_check_message): New functions.
(xgettext_check_message_list): Invoke url_check_message.
* gettext-tools/tests/xgettext-20: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add it.
* NEWS: Mention the change.