event: hook up sd-event with the service watchdog logic

Adds a new call sd_event_set_watchdog() that can be used to hook up the
event loop with the watchdog supervision logic of systemd. If enabled
and $WATCHDOG_USEC is set the event loop will ping the invoking systemd
daemon right after coming back from epoll_wait() but not more often than
$WATCHDOG_USEC/4. The epoll_wait() will sleep no longer than
$WATCHDOG_USEC/4*3, to make sure the service manager is called in time.

This means that setting WatchdogSec= in a .service file and calling
sd_event_set_watchdog() in your daemon is enough to hook it up with the
watchdog logic.

src/machine/machined.c

@@ -59,6 +59,8 @@ Manager *manager_new(void) {
                 return NULL;
         }
 
+        sd_event_set_watchdog(m->event, true);
+
         return m;
 }