Add hook to attach Upstart override files

As a follow-on to bug 803963, it would be great if apport could also attach any Upstart "override files" that have been created for a packages Upstart jobs.

Details
=====

Since the introduction of Upstart override files:

    http://upstart.ubuntu.com/cookbook/#ubuntu-natty-override-files

... it is now possible to create a "/etc/init/<job>.override" file that
can potentially completely change the behaviour of a service that is
controlled by Upstart.

As such, it would be great if apport were to look automatically attach
the associated ".override" file(s) to bug reports for a package.

Strategy should be to determine if any of the packages conffiles are
Upstart Job Configuration Files (files that match the pattern
"^/etc/init/.*\.conf") and if any of these have a corresponding
".override" file, attach the override file to the bug.

Assuming a package has a config file specified as "/etc/init/foo.conf",
in shell syntax...

  conf_file=/etc/init/foo.conf
  override_file=${conf_file/.conf/.override}

  if [ -e "$override_file" ]
  then
    # need to save packages override file corresponding to foo.conf
  fi