logind: automatically GC lingering users for who now user@.service (nor slice, not...

logind: automatically GC lingering users for who now user@.service (nor slice, not runtime dir service) is running anymore

This heavily borrows from @intelfx' PR #5546, but watches all three
units that are associated with a user now: the slice, the user@.service
and user-runtime-dir@.service.

The logic and reasoning behind it is the same though: there's no value
in keeping lingering users around if all their three services are gone.

Replaces: #5546
Fixes: #4162