Remove "internet-enabled" dmg creation.

Robby says that they're no longer recommended and rarely used by
browsers, and with a better dmg layout the contents should not be copied
automatically anyway.
Do the conventional DMG layout.

Note that this requires a running Finder.  Note also that the process
involves attaching the created DMG which means that it might be
problematic with a machine that might have a volume by the same name
already attached (since the applescript uses the volume name, which is
not required to be unique).
Make only binary distributions use laid-out DMGs.

This is done by adding an "ldmg" type for laid-out DMGs, and "dmg"
produces a plain container image as before.