[PATCH v2] doc: Define a standard URI syntax for NBD URIs.

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

Differences since v1:

- Make clear that we defer all questions about the NBD protocol
  or how it works to the NBD protocol document.

- Make clear that the userinfo field is only needed for TLS
  (not for NBD itself).

- `socket` parameter is still required, because I'm unhappy about
  a client opening a local file if no socket is specified (it's
  far more likely to be an error and opening a local file could
  raise security concerns).

- Add various tls parameters.  The most controversial(?) aspect of
  this is that I've gone with qemu-style certificates directory.

- Document how clients which do not support TLS should behave.

- Document that Unix domain sockets may begin with an ASCII NUL
  character on some platforms.

Rich.