nbd: Handle fixed new-style clients.

When this flag is set, the server tells the client that it can send another
option if the server received a request with an option that it doesn't
understand instead of directly closing the connection.

Also add link to the most up-to-date documentation.

Signed-off-by: Hani Benhabiles <kroosec@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>