MEDIUM: threads/signal: Add a lock to make signals thread-safe

A global lock has been added to protect the signal processing. So when a signal
it triggered, only one thread will catch it.
diff --git a/src/haproxy.c b/src/haproxy.c
index ff63844..81122c8 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -2819,6 +2819,10 @@
 		free(tids);
 		free(threads);
 
+#if defined(DEBUG_THREAD) || defined(DEBUG_FULL)
+		show_lock_stats();
+#endif
+
 #endif /* USE_THREAD */
 	}
 	else {