Update pstree source to point to a valid location

Freecode is no longer being updated, and their FTP seems to be down so downloading the code doesn't work.

http://www.thp.uni-duisburg.de/pstree/ is a valid host for the code. I changed the homepage to point to http://www.thp.uni-duisburg.de/pstree/ because the freshmeat page just redirects to the freecode page (which is stale and contains invalid download links).