feat: Add stbbr_for_presence_to for MUC join presence matching

- Add prime_for_presence_to() and prime_get_for_presence_to() in prime.c
- Add presence_to_callback() in server.c to handle MUC join presence
- Export stbbr_for_presence_to() in stabber.h
- Add stanzas_debug_last() for debugging failed verifications

stabber.h

@@ -38,6 +38,7 @@ void stbbr_set_timeout(int seconds);
 int stbbr_auth_passwd(char *password);
 int stbbr_for_id(char *id, char *stream);
 int stbbr_for_query(char *query, char *stream);
+int stbbr_for_presence_to(char *to_jid, char *stream);
 
 void stbbr_wait_for(char *id);