Previously logging in via SLiM more than once didn't work because SLiM
doesn't clean up its PAM session properly (that is, in a child rather
than in the parent). Thus the slim process becomes part of the user
session's cgroup, among other things. This patch causes SLiM to exit
after the session has finished, after which systemd will restart
display-manager.service.
FixesNixOS/nixops#137.
