logind: rework how we manage the slice and user-runtime-dir@.service unit for each...

logind: rework how we manage the slice and user-runtime-dir@.service unit for each user

Instead of managing it explicitly, let's simplify things and rely on
regular Wants=/Requires= dependencies to pull in these units from
user@.service and the session scope, and StopWhenUneeded= to stop these
auxiliary units again. This way, they can be pulled in easily by
unrelated units too.

This simplifies things quite a bit: for each session we now only need to
manage the session scope, and for each user the user@.service, the other
units are not something we need to manage anymore.

This patch also makes sure that if user@.service of a user is masked we
will continue to work, and user-runtime-dir@.service will still be
correctly pulled in, as it is now a dependency of the scope unit.

Fixes: #9461
Replaces: #5546