block/nbd: merge nbd-client.* to nbd.c

block/nbd: merge nbd-client.* to nbd.c

No reason for keeping driver handlers realization separate from driver
structure. We can get rid of extra header file.

While being here, fix comments style, restore forgotten comments for
NBD_FOREACH_REPLY_CHUNK and nbd_reply_chunk_iter_receive, remove extra
includes.

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-Id: <20190611102720[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Eric Blake <[email protected]>