debian/control: change Breaks statement to pre-4.0.1.21 x2goserver and add a Replaces: x2goserver << 4.0.1.21 section to x2goagent package.

We'll do the /etc/x2go/x2goagent.options file move for x2goserver
4.0.1.21 and nx-libs 3.5.0.33 already.