This was broken in 7468a10b6 (1.29.0), resulting in a missing diagnostics
and SSL error queue not cleared for SSL handshakes rejected by SNI, seen
as "ignoring stale global SSL error" alerts, for instance, when doing SSL
shutdown of a long standing connection after rejecting another one by SNI.

The fix is to move the qc->error check after c->ssl->handshake_rejected is
handled first, to make the error queue cleared.  Although not practicably
visible as needed, this is accompanied by clearing the error queue under
the qc->error case as well, to be on the safe side.

As an implementation note, due to the way of handling invalid transport
parameters for OpenSSL 3.5 and above, which leaves a passed pointer not
advanced on error, SSL_get_error() may return either SSL_ERROR_WANT_READ
or SSL_ERROR_WANT_WRITE depending on a library.  To cope with that, both
qc->error and c->ssl->handshake_rejected checks were moved out of
"sslerr != SSL_ERROR_WANT_READ".

Also, this reconstructs a missing "SSL_do_handshake() failed" diagnostics
for the qc->error case, replacing using ngx_ssl_connection_error() with
ngx_connection_error().  It is made this way to avoid logging at the crit
log level because qc->error set is expected to have an empty error queue.

Reported and tested by Vladimir Homutov.

A bug with the "quic_transport_parameters" extension and SNI described
in cedb855d7 is now fixed in the OpenSSL 3.5.1 release, as requested
in https://github.com/openssl/openssl/pull/27706.

In OpenSSL 3.5.0, the "quic_transport_parameters" extension set
internally by the QUIC API is cleared on the SSL context switch,
which disables sending QUIC transport parameters if switching to
a different server block on SNI.  See the initial report in [1].

This is fixed post OpenSSL 3.5.0 [2].  The fix is anticipated in
OpenSSL 3.5.1, which has not been released yet.  When building
with OpenSSL 3.5, OpenSSL compat layer is now used by default.
The OpenSSL 3.5 QUIC API support can be switched back using
--with-cc-opt='-DNGX_QUIC_OPENSSL_API=1'.

[1] https://github.com/nginx/nginx/issues/711
[2] https://github.com/openssl/openssl/commit/45bd3c3798

There were a few random places where 0 was being used as a null pointer
constant.

We have a NULL macro for this very purpose, use it.

There is also some interest in actually deprecating the use of 0 as a
null pointer constant in C.

This was found with -Wzero-as-null-pointer-constant which was enabled
for C in GCC 15 (not enabled with Wall or Wextra... yet).

Link: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117059>

Similarly to the QUIC API originated in BoringSSL, this API allows
to register custom TLS callbacks for an external QUIC implementation.
See the SSL_set_quic_tls_cbs manual page for details.

Due to a different approach used in OpenSSL 3.5, handling of CRYPTO
frames was streamlined to always write an incoming CRYPTO buffer to
the crypto context.  Using SSL_provide_quic_data(), this results in
transient allocation of chain links and buffers for CRYPTO frames
received in order.  Testing didn't reveal performance degradation of
QUIC handshakes, https://github.com/nginx/nginx/pull/646 provides
specific results.

Using SSL_in_init() to inspect a handshake state was replaced with
SSL_is_init_finished().  This represents a more complete fix to the
BoringSSL issue addressed in 22671b37e.

This provides awareness of the early data handshake state when using
OpenSSL 3.5 TLS callbacks in 0-RTT enabled configurations, which, in
particular, is used to avoid premature completion of the initial TLS
handshake, before required client handshake messages are received.

This is a non-functional change when using BoringSSL.  It supersedes
testing non-positive SSL_do_handshake() results in all supported SSL
libraries, hence simplified.

In preparation for using OpenSSL 3.5 TLS callbacks.

Encryption level values are decoupled from ssl_encryption_level_t,
which is now limited to BoringSSL QUIC callbacks, with mappings
provided.  Although the values match, this provides a technically
safe approach, in particular, to access protection level sized arrays.

In preparation for using OpenSSL 3.5 TLS callbacks.