[dwm] multiple columns diff

Hi, as mentioned in an earlier e-mail, on a xinerama setup with more two or more screens you might want more
than just the fixed "zoomed+stack" column layout. Attatched is a patch against dwm 0.9 which implements this.
It keeps a compile time constant number of columns (unless there are fewer clients than columns), but adds the extra facility to call a function to increment/decrement the division between full and stack columns; one useful thing that lets you do is temporarily move to "all stack columns", which can be useful.

It's proably not appropriate for the mainline because it's of limited use on single-display machines, but other people with similar setups might find it useful.
I've tried to test it in all degenerate conditions, but if you can break it please post to the list & I'll try and fix it.

cheers, dave tweed