cvsclient: Allow to connect to a pserver URL without password
The CVS client was raising an error when trying to connect to such pserver URL: pserver://anonymous@cvs.example.org/cvsroot/project/module
But numerous CVS pserver URLs that can be found in the wild (notably on SourceForge and OSDN) are in that form.
So add support for such URL form in the CVS client.
Migrated from D8004 (view on Phabricator)