event: when handling SIGCHLD of a child process only reap after dispatching event source

That way the even source callback is run with the zombie process still
around so that it can access /proc/$PID/ and similar, and so that it can
be sure that the PID has not been reused yet.
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.
man: explain in more detail how SYSTEMD_READY= influences SYSTEMD_WANTS= in udev rules

https://bugzilla.redhat.com/show_bug.cgi?id=1026860