cups/tls-gnutls.c: Use always GNUTLS_SHUT_WR

[zdohnal]

The current mode for gnutls_bye() in client use cases strictly
follows TLS v1.2 standard, which in this particular part says:

Unless some other fatal alert has been transmitted, each party is
required to send a close_notify alert before closing the write
side of the connection.  The other party MUST respond with a
close_notify alert of its own and close down the connection immediately,
discarding any pending writes.  It is not required for the initiator
of the close to wait for the responding close_notify alert before
closing the read side of the connection.

and waits for the other side of TLS connection to confirm the close.

Unfortunately it can undesired for reasons:

• we support switching of TLS versions in CUPS, and this mode strictly
  follows TLS v1.2 - so for older version this behavior is not expected
  and can cause delays
• even some TLS v1.2 implementations (like Windows Server 2016) don't
  comply TLS v1.2 behavior even if it says it does - in that case,
  encrypted printing takes 30s till HTTP timeout is reached, because the
  other side didn't send confirmation
• AFAIU openssl's SSL_shutdown() doesn't make this TLS v1.2 difference,
  so we could end up with two TLS implementations in CUPS which will
  behave differently

Since the standard defines that waiting for confirmation is not required
and due the problems above, I would propose using GNUTLS_SHUT_WR mode
regardless of HTTP mode.

[michaelrsweet]

OK.

[zdohnal]

Thank you for the review!