[dwm] Adapted the pertag pull request for 5.0.1

Greetings to all

This is my first attempt to improve-modify existing proprietary, so please
forgive any
imperfections or errors. I believe the pull request i send applied to 5.0.1 to be
free of any bugs.
It is a simplistic adaptation of the pertag pull request which was at the site to the
new version of dwm.
A friend asked me to write it so i worked with the existing pull request.
Hopefully the project maintainer can update the web site with this fixed
version.

Thanks in advance.