Revert "gnutls: Change the GnuTLS priority string from NORMAL to SECURE256"

On gnutls 2.12.x, SECURE256 just changes the preferred negotiation
order, but on 3.0.x, it means "disable all non-256-bit ciphers", which
breaks many sites.

This reverts commit 6550c7e9cf928b13ed1d1167004c1f6e16e0173f.
Updated Dutch translation
Updated Dutch translation
2.33.3
gnutls: use g_clear_object()
gnutls: preserve handshake errors

If an error occurs during handshake, remember it for later, since the
connection is now essentially broken.
gnutls: reorganize certificate handling

Move some of the code in handshake_internal() out into helper
functions, and move the code from the client and server verify_peer()
implementations into a helper function in gtlsconnection-gnutls.c as
well, to reduce duplication between them.
gnutls: handle simultaneous ops, do handshaking in a thread

Keep separate internal read/write states so that you can do
simultaneous reads/writes (either simultaneous async ops in the same
thread, or simultaneous sync ops in different threads). Add
locking/blocking so that this works correctly even in the presence of
rehandshakes (and add test cases for this).

As part of this, change handshaking so that the I/O part of it always
happens in a separate thread, which has three advantages:

  1. It simplifies GTlsConnectionGnutlsSource by making it not have to
     worry about flipping back and forth between read and write.

  2. (Future) It will let the caller asynchronously handle
     certificate-related functionality that is implemented via
     synchronous callbacks in gnutls.

  3. (Future) We can use g_task_set_return_on_cancel() to allow
     cancellation even during uncancellable PKCS#11 operations.

https://bugzilla.gnome.org/show_bug.cgi?id=656343
https://bugzilla.gnome.org/show_bug.cgi?id=660252
Updated Slovenian translation
Updated Spanish translation
Updated Lithuanian translation
Updated Norwegian bokmål translation
Updated Greek translation
Updated Hebrew translation.
Updated Serbian translation
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Updated Russian translation
Updated Traditional Chinese translation(Hong Kong and Taiwan)
gnutls: allow overriding the priority string with an environment variable

Based on a patch from Josh Rickmar

https://bugzilla.gnome.org/show_bug.cgi?id=677717
tls: Split the database tests out from the certificate tests

 * For now these test GTlsFileDatabase

https://bugzilla.gnome.org/show_bug.cgi?id=681119