Changed HOSTNAME and /etc/hosts handling.

Changed HOSTNAME and /etc/hosts handling.

* This is one of a series of interventions that means to be gentle to
  debian-live builders and to persistence users; the rationale is that
  if a file is present on the rootfs it does not need to be overwritten
  from scratch since its presence means either a live-helper
  configuration at build time or a persistent/snapshot at run time.