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.

src/haproxy-systemd-wrapper.c

@@ -123,6 +123,7 @@ static int read_pids(char ***pid_strv)
 
 static void signal_handler(int signum)
 {
+	if (caught_signal != SIGINT && caught_signal != SIGTERM)
 		caught_signal = signum;
 }