[PATCH v3] doc: Define a standard URI syntax for NBD URIs.

["Richard W.M. Jones" <rjones@redhat.com>]

v1: https://lists.debian.org/nbd/2019/05/msg00034.html

v2:
https://lists.debian.org/nbd/2019/05/msg00054.html
and continued here: https://lists.debian.org/nbd/2019/06/msg00001.html

v3:

- Document nbds+unix.

- Document tls-hostname parameter and what it's used for.

- Drop tls-* parameters related to certificates, paths.  We may (or
  may not) standardize them in future.

- Allow x-* parameters for local experiments, modelled after the same
  text in NBD protocol doc.

- Removed waffle about NBD servers and 7 bit export names, not
  necessary in a standards document.

- Fixed other minor problems found by various reviewers.

Rich.