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.h

@@ -0,0 +1,24 @@
+#ifndef CG_NOTIFY_H
+#define CG_NOTIFY_H
+
+#include "config.h"
+
+#define CAGE_ALIVE_PERIOD_MS (20000)
+
+enum cg_notify_state {
+	CAGE_READY,
+	CAGE_ALIVE,
+	CAGE_STOPPING,
+};
+
+#if !CAGE_HAS_SYSTEMD
+static inline void
+notify_set_state(enum cg_notify_state state)
+{
+	/* Nothing */
+}
+#else
+void notify_set_state(enum cg_notify_state state);
+#endif
+
+#endif