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>

The support first appeared in OS X Mavericks 10.9 and documented since
OS X Yosemite 10.10.

It has a subtle implementation difference from other operating systems
in that the TCP_KEEPALIVE socket option (used in place of TCP_KEEPIDLE)
isn't inherited from a listening socket to an accepted socket.

An apparent reason for this behaviour is that it might be preserved for
the sake of backward compatibility.  The TCP_KEEPALIVE socket option is
not inherited since appearance in OS X Panther 10.3, which long predates
two other TCP_KEEPINTVL and TCP_KEEPCNT socket options.

Thanks to Andy Pan for initial work.

Certain providers may attempt to reload the key on the first use after a
fork.  Such attempt would require re-prompting the pin, and this time we
are not able to pass the password callback.

While it is addressable with configuration for a specific provider, it would
be prudent to ensure that no such prompts could block worker processes by
setting the default UI method.

UI_null() first appeared in 1.1.1 along with the OSSL_STORE, so it is safe
to assume the same set of guards.

A new "store:..." prefix for the "ssl_certificate_key" directive allows
loading keys via the OSSL_STORE API.

The change is required to support hardware backed keys in OpenSSL 3.x using
the new "provider(7ossl)" modules, such as "pkcs11-provider".  While the
engine API is present in 3.x, some operating systems (notably, RHEL10)
have already disabled it in their builds of OpenSSL.

Related: https://trac.nginx.org/nginx/ticket/2449

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.

It is now called from ngx_quic_handle_crypto_frame(), prior to proceeding
with the handshake.  With this logic removed, the handshake function is
renamed to ngx_quic_handshake() to better match ngx_ssl_handshake().

All definitions now set in ngx_event_quic.h, this includes moving
NGX_QUIC_OPENSSL_COMPAT from autotests to compile time.  Further,
to improve code readability, a new NGX_QUIC_QUICTLS_API macro is
used for QuicTLS that provides old BoringSSL QUIC API.