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/network/networkd-manager.c

@@ -35,6 +35,8 @@ int manager_new(Manager **ret) {
         if (r < 0)
                 return r;
 
+        sd_event_set_watchdog(m->event, true);
+
         r = sd_rtnl_open(RTMGRP_LINK | RTMGRP_IPV4_IFADDR, &m->rtnl);
         if (r < 0)
                 return r;