lib/util: add debug_set_forced_log_priority()

lib/util: add debug_set_forced_log_priority()

By default the priority for syslog/systemd is derived from
the log level of the debug message.

But for things like startup messages we want to
change the priority temporary, like this:

debug_set_forced_log_priority(DBGLVL_NOTICE);
D_ERR("Startup...\n");
debug_set_forced_log_priority(-1);

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15377

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Björn Jacke <bjacke@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>