login: simply XDG_RUNTIME_DIR management

login: simply XDG_RUNTIME_DIR management

Lets not pretend we support changing XDG_RUNTIME_DIR via logind state
files. There is no reason to ever write the string into /run, as we
allocate it statically based on the UID, anyway. Lets stop that and just
allocate the runtime_path in "struct User" at all times.

We keep writing it into the /run state to make sure pam_systemd of
previous installs can still read it. However, pam_systemd is now fixed to
allocate it statically as well, so we can safely remove that some time in
the future.

Last but not least: If software depends on systemd, they're more than free
to assume /run/user/$uid is their runtime dir. Lets not require sane
applications to query the environment to get their runtime dir. As long as
applications know their login-UID, they should be safe to deduce the
runtime dir.