Comment 6 for bug 671190

So this probably needs something like:
 - a X- header in the dsc (to avoid race conditions)
 - -> mirrored into the datamodel (in SPPH ?)
 - which is permitted to refer to a chroot
 - do chroots need to be only provided by archive admins?
   * if so (because they can attack the buildd), then from a preset list
   * if not, then allow them to be attached to e.g. an Archive by anyone with edit