Add a --name command-line option for naming snapshots.

Add a --name command-line option for naming snapshots.

This feature is meant to support storing several separate sets of snapshots
in the same directory--for example, snapshots of multiple computers, or
snapshots of different sets of directories.  These snapshots can be deleted
independently.  The snapshots will be permitted to share segments.

A name is simply applied for management convience, to help in
distinguishing the different sets of backups.  The string specified with
--name is included both in the snapshot descriptor filename and as a
"Name:" field within that descriptor.