logind: add a RequiresMountsFor= dependency from the session scope unit to the home...

logind: add a RequiresMountsFor= dependency from the session scope unit to the home directory of the user

This is useful so that during shutdown scope units are always terminated
before the mounts necessary for the home directory.

(Ideally we'd also add a similar dependency from the user@.service
instance to the home directory, but this isn't as easy as that service
is defined statically and not dynamically, and hence not easy to modify
dynamically, in particular when it comes to deps)