[dwm] Small patch that allows user to select next and previous tags

Hi,

First of all, thanks for the great WM!

Anyway, I have small patch set that allows user to select tags
using Mod1-Left|Right (or whatever user wants to map them)
in case someone is interested. It is similar that was suggested
by Jukka Salmi in thread:

http://lists.suckless.org/dwm/0607/0026.html

but works against latest dwm sources.

Note that if user has several tags selected, only one
next/prev tag is then selected.

MW