Use &cgibase; rather than hardcoding http://www.freebsd.org/cgi/

There are a few remaining explicit uses of http://www.freebsd.org/cgi/
but all of these appear to be outdated e.g. pointing to GNATS) and need
updating.

Switch &cgibase; to use protocol-relative links, rather than hardcoding
http. Switch two others (ftpbase and ftpbaseerrata) whilst here.