logind-user: track user started/stopping state through user-runtime-dir@.service

logind-user: track user started/stopping state through user-runtime-dir@.service

Before #30884, the user state is tied to user@.service (user service
manager). However, #30884 introduced sessions that need no manager,
and we can no longer rely on that.

Consider the following situation:

1. A 'background-light' session '1' is created (i.e. no user service manager
   is needed)
2. Session '1' scope unit pulls in user-runtime-dir@.service
3. Session '1' exits. A stop job is enqueued for user-runtime-dir@.service
   due to StopWhenUnneeded=yes
4. At the same time, another session '2' which requires user manager is started.
   However, session scope units have JobMode=fail, therefore the start job
   for user-runtime-dir@.service that was pulled in by session '2' scope job
   is deleted as it conflicts with the stop job.

We want session scope units to continue using JobMode=fail, but we still need
the dependencies to be started correctly, i.e. explicitly requested by logind
beforehand. Therefore, let's stop using StopWhenUnneeded=yes for
user-runtime-dir@.service, and track users' `started` and `stopping` state
based on that when user@.service is not needed. Then, for every invocation
of user_start(), we'll recheck if we need the service manager and start it
if so.

Also, the dependency type on user-runtime-dir@.service from user@.service
is upgraded to `BindsTo=`, in order to ensure that when logind stops the
former, the latter is stopped as well.