logind: fix delayed execution regression

logind: fix delayed execution regression

Commit c0f32805 ("logind: use sd_event timer source for inhibitor
logic") reworked the main loop logic of logind so that it uses a
real timeout callback handler to execute delayed functions.

What the old code did, however, was to call those functions on
every iteration in the main loop, not only when the timeout
expired.

Restore that behavior by bringing back manager_dispatch_delayed(),
and call it from manager_run(). The internal event source callback
manager_inhibit_timeout_handler() was turned into a wrapper of
manager_dispatch_delayed() now.