gssd: Use /run/user/${UID} instead of /run/user/${USER}

gssd: Use /run/user/${UID} instead of /run/user/${USER}

Newer versions of systemd create a /run/user/${UID} directory
instead of the /run/user/${USER} directory, so switch to
scanning for that.  To make the per-user directory bit a little
less magical, change the default to incorporate a "%U", which
gets dynamically expanded to the user's UID when needed.

Signed-off-by: Steve Dickson <steved@redhat.com>