[dwm] Quick patch for multihead

Hello,

As I have no time to code, I have written this patch in 30 minutes. It's a
patch to have a multi-head support "like" E17, I am not at home, so I don't
know if it works correctly ! The only thing I know for sure is that it
compiles without error on a PC which is not mine. Maybe it won't work at
all. I won't be in front of my PC until tonight so can you (if you have
nothing else to do) test it ?

(I have set MODKEY to Mod4Mask)

MODKEY + AltKey + Number = Place the select client on the screen number
Number, if a client is already on the screen, it is replaced.
MODKEY + Enter = normal behavior but when the window is on another screen it
moves it in the main screen (where the bar is)

Thanks