[dwm] dwm-1.2 large numbers of clients diff (includes xinerama)

Hi, attached is latest version of my large numbers of clients patch.
The only change wrt version for dwm-1.1 is it now again
`wraps around' the display on 2 monitor xinerama setups in
an attempt to put zoomed column closer to middle. (See patchDoc.txt
for how to disable.) This has the effect of making the code a bit more
ugly (since integer pixel rounding errors mean the obvious modulo algorithm
doesn't always work). However, I decided I really don't like the main
column alwyas being on the far left.

Working on making some screenshots available as requested.

cheers, dave tweed