service: don't warn if the pidfile still exists after SIGCHLD

A service that drops its privileges may not be able to remove it when it
exits. The stale pidfile is not a problem as long as the service
carefully recognizes it on its next start.

systemd would produce a warning after the service exits:
  PID ... read from file ... does not exist. Your service or init
  script might be broken.

Silence the warning in this case. Still warn if this error is detected
when loading the pidfile after service start.

Noticed by Miroslav Lichvar in
 https://bugzilla.redhat.com/show_bug.cgi?id=752396