Add bupdrecurse.py

bupdrecurse is an implementation of drecurse that runs on the bup VFS
layer and returns bup metadata objects.

This allows conversion of stored bup backups back into bupindex files as
used by the client.
Use shlex to parse the bup-server command line.

Adds the Python shlex library for parsing the bup-server command line.
This enables quoted filenames to be passed to commands.
Intermediate commit for bup-server remote-restore and test protocol.