* export adt_* variable settings resulting from command-line

  arguments so that subprocesses get them properly, and arrange
  for the `defaults for simple settings' not to override environment
  variables.
* suppress various variable settings' default computations (and
  consequent failure if the default can't be established) when
  processing config for the initscript.  The initscript does not
  need per-testbed configuration options.
* close fd 8 when running vgdisplay (which avoids an annoying and
  spurious warning from the lvm tools).