Add version information to the backup descriptor files.

Add version information to the backup descriptor files.

This adds a "Format:" line at the start of a backup descriptor, with the
intent that different formats can be recognized if there is a need to make
changes in the future.  This can also be used as a magic number to identify
LBS snapshot files.

Also, update the restore.pl script to handle the new format--make the
parser more flexible so that fields can appear in any order, and
unrecognized lines are ignored.