Before doing anything, make a copy of loader.conf to loader.conf.preemulab.
Have prepare script copy back loader.conf.preemulab before creating a new
image. This will hopefully get rid of node-specific cruft that we put into
that file at slicefix time (e.g., disabling interfaces, changing the
console device). We also just append all our variable settings to the end
of that file rather than trying to clean out other settings or change them
in place. This allows another potential form of cleanup--just remove
everything after a certain point in the file.

This should conclude (modulo bug fixes) my recent unhealthy obsession with
rc.frisbee/slicefix...