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 <vsementsov@virtuozzo.com>
Message-Id: <20190611102720.86114-3-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

block/Makefile.objs

@@ -22,7 +22,7 @@ block-obj-y += null.o mirror.o commit.o io.o create.o
 block-obj-y += throttle-groups.o
 block-obj-$(CONFIG_LINUX) += nvme.o
 
-block-obj-y += nbd.o nbd-client.o
+block-obj-y += nbd.o
 block-obj-$(CONFIG_SHEEPDOG) += sheepdog.o
 block-obj-$(CONFIG_LIBISCSI) += iscsi.o
 block-obj-$(if $(CONFIG_LIBISCSI),y,n) += iscsi-opts.o