Similar to map's volatile parameter, creates a non-cacheable geo variable.

Variables contain the IANA name of the signature scheme[1] used to sign
the TLS handshake.

Variables are only meaningful when using OpenSSL 3.5 and above, with older
versions they are empty.  Moreover, since this data isn't stored in a
serialized session, variables are only available for new sessions.

[1] https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml

Requested by willmafh.

After f10bc5a763bb the address was set to NULL only when local address was
not specified at all.  In case complex value evaluated to an empty or
invalid string, local address remained unchanged.  Currenrly this is not
a problem since the value is only set once.  This change is a preparation
for being able to change the local address after initial setting.

OCSP response in TLSv1.3 is sent in the Certificate message.  This
is incompatible with pre-compression of the configured certificates.

This brings feature parity with OpenSSL after the previous change,
making it possible to set SSL protocols per virtual server.

The change introduces an SNI based virtual server selection during
early ClientHello processing.  The callback is available since
OpenSSL 1.1.1; for older OpenSSL versions, the previous behaviour
is kept.

Using the ClientHello callback sets a reasonable processing order
for the "server_name" TLS extension.  Notably, session resumption
decision now happens after applying server configuration chosen by
SNI, useful with enabled verification of client certificates, which
brings consistency with BoringSSL behaviour.  The change supersedes
and reverts a fix made in 46b9f5d38 for TLSv1.3 resumed sessions.

In addition, since the callback is invoked prior to the protocol
version negotiation, this makes it possible to set "ssl_protocols"
on a per-virtual server basis.

To keep the $ssl_server_name variable working with TLSv1.2 resumed
sessions, as previously fixed in fd97b2a80, a limited server name
callback is preserved in order to acknowledge the extension.

Note that to allow third-party modules to properly chain the call to
ngx_ssl_client_hello_callback(), the servername callback function is
passed through exdata.

The ssl_certificate_compression directive allows to send compressed
server certificates.  In OpenSSL, they are pre-compressed on startup.
To simplify configuration, the SSL_OP_NO_TX_CERTIFICATE_COMPRESSION
option is automatically cleared if certificates were pre-compressed.

SSL_CTX_compress_certs() may return an error in legitimate cases,
e.g., when none of compression algorithms is available or if the
resulting compressed size is larger than the original one, thus it
is silently ignored.

Certificate compression is supported in Chrome with brotli only,
in Safari with zlib only, and in Firefox with all listed algorithms.
It is supported since Ubuntu 24.10, which has OpenSSL with enabled
zlib and zstd support.

The actual list of algorithms supported in OpenSSL depends on how
the library was configured; it can be brotli, zlib, zstd as listed
in RFC 8879.

Passwords were not preserved in optimized SSL contexts, the bug had
appeared in d791b4aab (1.23.1), as in the following configuration:

    server {
        proxy_ssl_password_file password;
        proxy_ssl_certificate $ssl_server_name.crt;
        proxy_ssl_certificate_key $ssl_server_name.key;

        location /original/ {
            proxy_pass https://u1/;
        }

        location /optimized/ {
            proxy_pass https://u2/;
        }
    }

The fix is to always preserve passwords, by copying to the configuration
pool, if dynamic certificates are used.  This is done as part of merging
"ssl_passwords" configuration.

To minimize the number of copies, a preserved version is then used for
inheritance.  A notable exception is inheritance of preserved empty
passwords to the context with statically configured certificates:

    server {
        proxy_ssl_certificate $ssl_server_name.crt;
        proxy_ssl_certificate_key $ssl_server_name.key;

        location / {
            proxy_pass ...;

            proxy_ssl_certificate example.com.crt;
            proxy_ssl_certificate_key example.com.key;
        }
    }

In this case, an unmodified version (NULL) of empty passwords is set,
to allow reading them from the password prompt on nginx startup.

As an additional optimization, a preserved instance of inherited
configured passwords is set to the previous level, to inherit it
to other contexts:

    server {
        proxy_ssl_password_file password;

        location /1/ {
            proxy_pass https://u1/;
            proxy_ssl_certificate $ssl_server_name.crt;
            proxy_ssl_certificate_key $ssl_server_name.key;
        }

        location /2/ {
            proxy_pass https://u2/;
            proxy_ssl_certificate $ssl_server_name.crt;
            proxy_ssl_certificate_key $ssl_server_name.key;
        }
    }

It appears to be a relic from prototype locking removed in b0b7b5a35.