[dev] [dwm] pertag and bstack for 5.6 release

Hi there,

First of all, I got to say thank you to Anselm and the contributors to
dwm, which truly is a wonder.

I just switched to release 5.6, and ported a couple of patches that I
happen to use : bstack and pertag.

I don't have a dual head setup to test everything, but in a few hours
of usage I haven't noticed any problem with these so far, so I guess I
can share them.
Many thanks to the original authors of these patches.

PS: my C kungfu is a bit rusty and there is a compiler warning I haven't
taken time to fix with the pertag patch. Boo-boo me at will.

-- 
Francois Gombault