Refactor xrdp_listen to allow for privilege drop

- xrdp_listen.c is refactored so we can create the
  listening socket(s) before dropping privileges.
- The code which reads startup params from xrdp.ini
  is moved from xrdp_listen.c to xrdp.c, so it
  is only called once if we test the listen before
  starting the daemon.

common/thread_calls.c

@@ -122,8 +122,11 @@ tc_mutex_delete(tbus mutex)
     pthread_mutex_t *lmutex;
 
     lmutex = (pthread_mutex_t *)mutex;
-    pthread_mutex_destroy(lmutex);
-    g_free(lmutex);
+    if (lmutex != NULL)
+    {
+        pthread_mutex_destroy(lmutex);
+        g_free(lmutex);
+    }
 #endif
 }