[dwm] dwm-1.0 large numbers of clients diff (includes xinerama)

Hi, attatched is the diff against dwm-1.0 of both my multiple columns pull request
and a couple of other features I find useful for dealing with lots of clients.
Note that whilst this doesn't specifically deal with xinerama (eg, there's no
logic to stop windows spanning the join between two monitors), I
primarily wrote it to make dwm even less unusable on my xinerama setup
where you've got essentially a much bigger and wider screen.

There's new file pull requestDoc.txt generated by the diff that gives a very
description of what this pull request provides.

As usual, if you spot any errors in this stuff let me know and I'll try to
fix it,

cheers, dave tweed