Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2020-10-08-v3' into staging

nbd patches for 2020-10-08

- silence compilation warnings
- more fixes to prevent reconnect hangs
- improve 'qemu-nbd' termination behavior
- cleaner NBD protocol compliance on string handling

# gpg: Signature made Fri 09 Oct 2020 21:05:30 BST
# gpg:                using RSA key 71C2CC22B1C4602927D2F3AAA7A16B4A2527436A
# gpg: Good signature from "Eric Blake <eblake@redhat.com>" [full]
# gpg:                 aka "Eric Blake (Free Software Programmer) <ebb9@byu.net>" [full]
# gpg:                 aka "[jpeg image of size 6874]" [full]
# Primary key fingerprint: 71C2 CC22 B1C4 6029 27D2  F3AA A7A1 6B4A 2527 436A

* remotes/ericb/tags/pull-nbd-2020-10-08-v3:
  nbd: Simplify meta-context parsing
  nbd/server: Reject embedded NUL in NBD strings
  qemu-nbd: Honor SIGINT and SIGHUP
  block/nbd: nbd_co_reconnect_loop(): don't connect if drained
  block/nbd: fix reconnect-delay
  block/nbd: correctly use qio_channel_detach_aio_context when needed
  block/nbd: fix drain dead-lock because of nbd reconnect-delay
  nbd: silence maybe-uninitialized warnings

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>