[dwm] bstack improvements

Hello,

While using bstack, a patch made available on suckless.org, I noticed
that the gap between the main window and the horizontally stacked
windows was not being filled (in the case that resize hints made the
main window smaller). Additionally, the gaps between the horizontally
stacked windows were not being filled.

I updated my copy of bstack.c in light of these concerns and have
attached a copy to share.

-- 
Eric Mertens