MDEV-34129 mariadb-install-db appears to hang on macOS

Immediately close down the signal handler loop when we decide to
break connections as it's the start of process termination
anyway, and there's no need to wait once we've invoked break_connections.

sql/mysqld.cc

@@ -2953,6 +2953,15 @@ static void start_signal_handler(void)
   DBUG_VOID_RETURN;
 }
 
+/** Called only from signal_hand function. */
+static void* exit_signal_handler()
+{
+    my_thread_end();
+    signal_thread_in_use= 0;
+    pthread_exit(0);  // Safety
+    return nullptr;  // Avoid compiler warnings
+}
+
 
 /** This threads handles all signals and alarms. */
 /* ARGSUSED */
@@ -3013,10 +3022,7 @@ pthread_handler_t signal_hand(void *arg __attribute__((unused)))
     if (abort_loop)
     {
       DBUG_PRINT("quit",("signal_handler: calling my_thread_end()"));
-      my_thread_end();
-      signal_thread_in_use= 0;
-      pthread_exit(0);				// Safety
-      return 0;                                 // Avoid compiler warnings
+      return exit_signal_handler();
     }
     switch (sig) {
     case SIGTERM:
@@ -3035,6 +3041,7 @@ pthread_handler_t signal_hand(void *arg __attribute__((unused)))
         PSI_CALL_delete_current_thread();
         my_sigset(sig, SIG_IGN);
         break_connect_loop(); // MIT THREAD has a alarm thread
+        return exit_signal_handler();
       }
       break;
     case SIGHUP: