mxqd: Dump server state on SIGUSR2 q=10

A dump of server state can be triggerd by

  env kill -usr2 -q 10 mxqd

mxqd.c

@@ -1364,9 +1364,6 @@ void server_dump(struct mxq_server *server)
     struct mxq_group *group;
     struct mxq_job   *job;
 
-    if (!server->user_cnt)
-        return;
-
     mx_log_info("====================== SERVER DUMP START ======================");
     for (ulist = server->users; ulist; ulist = ulist->next) {
         if (!ulist->groups) {
@@ -2300,6 +2297,17 @@ static void process_signal(struct mxq_server *server,int sig,int extra)
             mx_log_info("received sigusr2");
             global_sigrestart_cnt++;
             break;
+        case SIGUSR2:
+            switch (extra) {
+                case 10:
+                    mx_log_info("received sigusr2 extra %d (dump)",extra);
+                    server_dump(server);
+                    break;
+                default:
+                    mx_log_warning("received sigusr2 extra %d (unexpected!)",extra);
+                    break;
+            }
+            break;
         case SIGCHLD:
             mx_log_info("received sigchld");
             break;