[dwm] Yet another simplification, this time sel/prevtags

Hi,

attached pull request makes one array for both sel and prevtags. So there is
no need to swap them in memory in view() as we only change seltags
pointer to point viewtags[0] or viewtags[1].

It also with little change can support arbitrary number of saved tags
set (3 save position can be unusable, with modkey-shift-space going on
"next" state in queue).

-Ph