Explore if nbd_aio_notify_error is needed for faster response if
server goes away.

Bindings in other languages.

Example code integrating with ppoll, pollfd, APR pollset (and others?).

Example command line utils to copy in/out (like qemu-img convert).

NBD_OPT_INFO mode (like qemu-nbd -L).

NBD resize extension.

NBD_INFO_BLOCK_SIZE.

TLS should properly shut down the session (calling gnutls_bye).

Implement nbd_connect + systemd socket activation.

Improve function trace output so that:
 - Long strings are truncated.
 - Strings with non-printable characters are escaped.
 - Buffers are partially printed.
 - Lists of strings are printed.

Suggested API improvements:
  general:
  - synchronous APIs that have a timeout or can be cancelled

  connecting:
  - nbd_connect_tcp: allow control over whether IPv4 or IPv6 is desired
  - nbd_connect_uri: allow control over which features are enabled
  - nbd_connect_command: allow passing char **env

  TLS:
  - should be individual APIs for setting each TLS file
    (set_tls_certificates can continue to exist)
  - TLS should have a way to pass in PIN or password to
    support encrypted keys
  - allow use of TLS memory APIs (gnutls_datum_t + gnutls_*_mem)
  - support PKCS11 URIs (RFC 7512)
