Brush up a bit the Ports CVSup method:

• We currently state "Install the net/cvsup port"; let's point directly to the package, in particular cvsup-without-gui since all commands, in the text, used the legacy "-g"...
• Most of people do not edit the ports-supfile and directly use the -h argument for the CVSup server, the reader should uses/knows that.
• Keep, as note, the ports-supfile edition for a "hardcoded" CVSup server
• I kept the odd <link linkend/<xref linkend way, maybe this should be fixed

The text should, now, be closer to what we do when it's time to CVSup
the Ports tree.