Rename check-for-reboot; save unity's stamp file

- check-for-reboot is now x-logout-tasks to better reflect what it
  does.
- We install a compatibility symlink with the old name so we don't
  break people who copied the system bashrc/cshrc.
- x-logout-tasks saves unity's first-run stamp file in the user's
  homedir, so we can refer to it later.