[Libguestfs] [PATCH nbdkit RFC 0/2] curl: Implement authorization scripts.

This is an RFC only, at the very least it lacks tests. This implements a rather complex new feature in nbdkit-curl-plugin allowing you to specify an external shell script that can be used to fetch an authorization token for services which requires a token or cookie for access, especially if that token must be renewed periodically. The motivation can be seen in the changes to the docs in patch 2. Rich.