New command "bup nbd"

Export Files (or "directories" resulting from "bup join") as network-block-devices.

This allows f.e. mounting disk-images saved by "bup join".

Signed-off-by: Tim Riemenschneider <git@tim-riemenschneider.de>
Documentation for bup-nbd

Some documentation for bup-nbd with usage-examples


Signed-off-by: Tim Riemenschneider <git@tim-riemenschneider.de>
Add some inline documentation to the code

Signed-off-by: Tim Riemenschneider <git@tim-riemenschneider.de>