Comment 21 for bug 8634

Some thoughts I elaborated on IRC, but haven't heard back about:
  * When writing out file, write out '## This file written by dhcp3-client', or
the traditional '## OMGDEBCONFBONG' markers.
  * Don't molest file if it's present, sans these markers.
  * If it (the file) is present, only molest it if markers are present -- either
replacing or appending, depending on whether we run with the former or latter
marker option.
  * Should make pon do something similar.

Matt, thoughts? I'm happy to claim this one.