[dwm] rfc: patch: xinerama support

Here's a first attempt at working Xinerama for dwm. It has rough edges
(and probably sharp corners), but perhaps someone else will find it
interesting and provide feedback.

The current code seems very confused about the difference between
Zaphod and Xinerama. I cleaned this up only where absolutely necessary
so far, but this has probably made Zaphod mode support more difficult
than it was.

Tags are per-display rather than per-monitor. This fit more naturally
with the way I work (tested by playing with awesome and xmonad),
though it is somewhat arbitrary. If two monitors display the same tag
then the client appears on the lowest numbered monitor (again
arbitrary).

There are changes to config.def.h - don't forget to merge them into
your config.h as appropriate.

I'll do more work on this, but wanted to get early feedback.

Comments?

dme.