nbd: Add types for extended headers

nbd: Add types for extended headers

Add the constants and structs necessary for later patches to start
implementing the NBD_OPT_EXTENDED_HEADERS extension in both the client
and server, matching recent upstream nbd.git (through commit
e6f3b94a934).  This patch does not change any existing behavior, but
merely sets the stage for upcoming patches.

This patch does not change the status quo that neither the client nor
server use a packed-struct representation for the request header.
While most of the patch adds new types, there is also some churn for
renaming the existing NBDExtent to NBDExtent32 to contrast it with
NBDExtent64, which I thought was a nicer name than NBDExtentExt.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
---

v5: Add R-b

v4: Hoist earlier in series, tweak a few comments, defer docs/interop
change to when feature is actually turned on, NBDExtent rename, add
QEMU_BUG_BUILD_ON for sanity sake, hoist in block status payload bits
from v3 14/14; R-b dropped