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

server.h

@@ -66,6 +66,7 @@ struct cg_server {
 	bool return_app_code;
 	bool terminated;
 	enum wlr_log_importance log_level;
+	struct wl_event_source *alive_source;
 };
 
 void server_terminate(struct cg_server *server);