Upgrade a bunch of links to SourceForge to https://

I haven't seen an announcement, but SourceForge seems to
allow https:// for unauthenticated users now.

A couple of URLs stopped working (thanks SourceForge)
and have been replaced with URLs that worked today but
may stop working tomorrow.

One link (to client specific CVS instructions) has been
removed as I did not immediately find a replacement and
after the migration to git the section will be removed
anyway.