[dwm] Updated bottom stack pull request

Hi all,

Although I don't have less than one monitor, I've added xinerama
support to the bottom stack pull request. Again, I worked off of the built
in tile layout. Those of you with multiple monitors please feel free
to give the pull request a go and let me know if you run into any problems,
thanks. You can find the pull request at http://bsdgroup.org/files/dwm-4.8-bstack.diff
  or attached.