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/timedate/timedated.c

@@ -836,6 +836,8 @@ int main(int argc, char *argv[]) {
                 goto finish;
         }
 
+        sd_event_set_watchdog(event, true);
+
         r = connect_bus(&context, event, &bus);
         if (r < 0)
                 goto finish;