[dwm] "pile" layout for dwm-4.0

Hello,

I'm switching from larswm to dwm and I made an attempt at porting the
features I use most.

So, here's the "pile" layout, which is a tiled layout where the tiles
are not resized to fit in screen - windows are just moved to the
right and stacked like a pile of cards. There's an option to choose
whether the top-left ("piletop" layout) or bottom-left ("pilebottom")
corner of tiled windows should be left visible. I use the bottom
variant, since it keeps the status lines of mutt, emacs and shell
prompts on screen.

Attached is the patch against dwm-4.0 sources.

Kudos to Anselm R. Garbe for the very clean code: it was easier to
port "pilebottom" to dwm than to write the original patch for larswm;
also, thanks to Ross Mohn for the bstack patch, which I took as a
starting point.

Ciao,
Riccardo