Override to xfce for kfreebsd and hurd. This was previously done in several
different places, including debian-cd and d-i boot parameters.

tasksel/desktop no longer has a default value, instead
/usr/lib/tasksel/default_desktop is used to look it up for an architecture.
This shell library might also be used by eg, debian-cd.

(It would also be ok to make tests/default-desktop vary the default based on
eg, machine hardware (ie, for a tablet, or a machine without much ram).)