[MINOR] support a global jobs counter

This counter is incremented for each incoming connection and each active
listener, and is used to prevent haproxy from stopping upon SIGUSR1. It
will thus be possible for some tasks in increment this counter in order
to prevent haproxy from dying until they have completed their job.
diff --git a/src/proxy.c b/src/proxy.c
index 346d0b3..f25216c 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -615,6 +615,7 @@
 		if (l->state >= LI_ASSIGNED) {
 			delete_listener(l);
 			listeners--;
+			jobs--;
 		}
 	}
 	p->state = PR_STSTOPPED;