MINOR: systemd: ensure a reload doesn't mask a stop

If a SIGHUP/SIGUSR2 is sent immediately after a SIGTERM/SIGINT and
before wait() is notified, it will mask it since there's no queue,
only a copy of the last received signal. Let's add a special check
before overwriting the signal so that SIGTERM/SIGINT are not masked.
diff --git a/src/haproxy-systemd-wrapper.c b/src/haproxy-systemd-wrapper.c
index 0c076a6..d118ec6 100644
--- a/src/haproxy-systemd-wrapper.c
+++ b/src/haproxy-systemd-wrapper.c
@@ -123,7 +123,8 @@
 
 static void signal_handler(int signum)
 {
-	caught_signal = signum;
+	if (caught_signal != SIGINT && caught_signal != SIGTERM)
+		caught_signal = signum;
 }
 
 /* handles SIGUSR2 and SIGHUP only */