[Nbd] proposal

[Folkert van Heusden <folkert@...421...>]

Hi,

I propose two new commands:

- NBD_SERVER_INFO
  which retrieves a textual description of the server it is connected
to. this then can for example contain information like buffer
settings, parameters, version of the software used, etc.

- NBD_ELABORATE_ON_ERROR
 if a command fails, an error code is returned. afaik this is an errno
value. sometimes you want to elaborate on this. for example a
server-device which detected that the datastructures got invalid or so
me return EINVALID (or whatever) and in response to this command a
textual description of what is wrong: "datastore corrupt, run fsck"