Change default backup filename and terminology

We now use "backup.ab" as the default backup archive filename, and no longer
refer to "tar" or "tarfiles" in the help text.  The underlying format may
be tar, but we're certainly not interoperable even with ustar/pax thanks
to our compression & encryption layers and our custom header.

Change-Id: I3e74af96cfc102e94848c969eb36af54304bfd9b