Lots of improvements to DMG creation.

* Remove lots of sudoing, and instead use one to change the contents (of
  non-simple DMGs).  Perhaps that's not needed too.  (I think that there
  used to be some problem when the owner was a specific user in the
  installer script days, perhaps it caused some mess in the pre/post
  flight scripts.  Maybe it's safe to not do this anymore.)

* There was a fundamental weakness in the Finder script -- it tweaks the
  DMG by openning its mount, but the way to specify a disk is via its
  displayed name which is not unique.  This can be problematic if there
  is already a mount that has the same name, and given that the DMG
  machine can be used by PLT people, it's even likely to happen.  It is
  tricky to solve this, since there is no way that I see to refer to the
  disk by its mount point or device in the Finder, and no way to find
  out the name in the shell.

  I spent a ton of time searching the webs for a solution, but it seems
  that practically nobody is concerned (or even aware of this problem).
  Eventually, the hack I did is to mount the image onto a randomly named
  directory, then iterate over all disks and look for one whose `name'
  property is that directory name (apparently the `name' property is the
  name of the last component of the mount point).  For safety, throw an
  error if no such disk is found, or if more than one is found.

  Incidentally, using a known mount point also simplifies things since
  there is no need to know the device name of the mount.

* Use UDBZ instead of UDZO for the compressed image.  The savings are
  not as big as I expected them to be for some reason, but the
  compatibility issues should be all gone now (UDBZ works only from 10.4
  and up).

* Unset $LD_LIBRARY_PATH to avoid some 10.8 bug.

* Switch the build to happen on dublin, running 10.8.  At least
  code-signing is broken on the 10.5.8 machine, and possibly also the
  osascript to identify the desired image disk.