common: Add generic status print function.

* common/status.c (gnupg_set_status_fd): New.
(gnupg_status_printf): New.
* po/Makevars (XGETTEXT_OPTIONS): Add gnupg-status_printf.

Some of the extra tools take a --status-fd option to print certain
status messages. A generic printf style print function thus makes
sense.

• Signed-off-by: Werner Koch <wk@gnupg.org>