Genericize 'logout' alias for X sessions

[jdreed]

Right now, there's a bunch of stuff hardcoded in bashrc/cshrc. We should genericize this, so that logout() merely calls /usr/lib/debathena-xsession/xlogout.sh or something, which can then do whatever we want. We currently have a hook (in the dotfiles) for checking for reboots. We should move that to the xsession package, and also add a snippet that can deal with things like the Unity XDG_CACHE_HOME bug.