login: initialize XDG_RUNTIME_DIR as /run/user/$UID instead of /run/user/$USER

login: initialize XDG_RUNTIME_DIR as /run/user/$UID instead of /run/user/$USER

The sssd folks would like to place the kerberos credential cache in
XDG_RUNTIME_DIR, but need to do that in the PAM auth hooks, before
pam_systemd is run as part of the PAM session setup. Hence, in order to
make this easy for them: avoid usage of usernames, and use user IDs
instead thus making an additional NSS lookup unnecessary in the kerberos
bits, but still have the directory well-defined so that the kerberos
bits can determine it before pam_systemd is run.