Add a restore-snapshot command to lbs-util.

Add a restore-snapshot command to lbs-util.

This is still in development, and not fully tested yet.  This is meant to
mostly replace restore.pl, and will eventually have more features (it can
already restore without needing all segments unpacked first).

The restore.pl script won't go away, though, since it is still useful to
include as a small, self-contained bare-bones restore program.