automount: make sure the expire event is restarted after a daemon-reload (#4265)

automount: make sure the expire event is restarted after a daemon-reload (#4265)

If the corresponding mount unit is deserialized after the automount unit
then the expire event is set up in automount_trigger_notify(). However, if
the mount unit is deserialized first then the automount unit is still in
state AUTOMOUNT_DEAD and automount_trigger_notify() aborts without setting
up the expire event.
Explicitly call automount_start_expire() during coldplug to make sure that
the expire event is set up as necessary.

Fixes #4249.