BUG/MEDIUM: mworker: unregister the signals of main()

The signal_register_fct() does not remove the handlers assigned to a
signal, but add a new handler to a list.

We accidentality inherited the handlers of the main() function in the
master process which is a problem because they act on the proxies.

The side effect was to stop the MASTER proxy which handle the master CLI
on a SIGUSR1, and to display some debug info when doing a SIGHUP and a
SIGQUIT.
diff --git a/src/haproxy.c b/src/haproxy.c
index 6a1397a..79de01b 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -862,6 +862,10 @@
 
 	master = 1;
 
+	signal_unregister(SIGUSR1);
+	signal_unregister(SIGHUP);
+	signal_unregister(SIGQUIT);
+
 	signal_register_fct(SIGTERM, mworker_catch_sigterm, SIGTERM);
 	signal_register_fct(SIGUSR1, mworker_catch_sigterm, SIGUSR1);
 	signal_register_fct(SIGINT, mworker_catch_sigterm, SIGINT);