Add optional feature to notify status to systemd

When enabled, the feature uses libsystemd to notify service manager
about the following states:
- ready
- periodic alive ping (20s)
- stopping

It allows to use Cage in a systemd unit with following options:
Type=notify
WatchdogSec=30s

notify_systemd.c

@@ -0,0 +1,33 @@
+/*
+ * Cage: A Wayland kiosk.
+ *
+ * Copyright (C) 2018-2020 Jente Hidskes
+ *
+ * See the LICENSE file accompanying this file.
+ */
+
+#include "config.h"
+
+#include <systemd/sd-daemon.h>
+
+#include "notify.h"
+
+void
+notify_set_state(enum cg_notify_state state)
+{
+	const char *sd_state;
+
+	switch (state) {
+	case CAGE_READY:
+		sd_state = "READY=1";
+		break;
+	case CAGE_ALIVE:
+		sd_state = "WATCHDOG=1";
+		break;
+	case CAGE_STOPPING:
+		sd_state = "STOPPING=1";
+		break;
+	}
+
+	sd_notify(0, sd_state);
+}