Server flags are not yet set (or used) correctly.

[Merovius]

The server should derive correct flags from the optional interfaces the Device supports (as well as the features we have implemented).

Relevant spec values:

• NBD_FLAG_HAS_FLAGS (should always be set)
• NBD_FLAG_READ_ONLY (make WriteAt optional?)
• NBD_FLAG_SEND_FLUSH (make Sync optional?)
• NBD_FLAG_SEND_FUA (#2)
• NBD_FLAG_ROTATIONAL (#11)
• NBD_FLAG_SEND_TRIM (#7)
• NBD_FLAG_SEND_WRITE_ZEROES (#9)
• NBD_FLAG_SEND_DF (#6)
• NBD_FLAG_CAN_MULTI_CONN (should always be set)
• NBD_FLAG_SEND_RESIZE (ignore, probably?)
• NBD_FLAG_SEND_CACHE (#12)