A new directive "ssl_certificate_cache max=N [valid=time] [inactive=time]"
enables caching of SSL certificate chain and secret key objects specified
by "ssl_certificate" and "ssl_certificate_key" directives with variables.

Co-authored-by: Aleksei Bavshin <a.bavshin@nginx.com>

It has been deprecated since 7270:46c0c7ef4913 (1.15.0) in favour of
the "ssl" parameter of the "listen" directive, which has been available
since 2224:109849282793 (0.7.14).

In some cases it might be needed to reject SSL handshake based on SNI
server name provided, for example, to make sure an invalid certificate
is not returned to clients trying to contact a name-based virtual server
without SSL configured.  Previously, a "ssl_ciphers aNULL;" was used for
this.  This workaround, however, is not compatible with TLSv1.3, in
particular, when using BoringSSL, where it is not possible to configure
TLSv1.3 ciphers at all.

With this change, the ssl_reject_handshake directive is introduced,
which instructs nginx to reject SSL handshakes with an "unrecognized_name"
alert in a particular server block.

For example, to reject handshake with names other than example.com,
one can use the following configuration:

    server {
        listen 443 ssl;
        ssl_reject_handshake on;
    }

    server {
        listen 443 ssl;
        server_name example.com;
        ssl_certificate example.com.crt;
        ssl_certificate_key example.com.key;
    }

The following configuration can be used to reject all SSL handshakes
without SNI server name provided:

    server {
        listen 443 ssl;
        ssl_reject_handshake on;
    }

    server {
        listen 443 ssl;
        server_name ~^;
        ssl_certificate example.crt;
        ssl_certificate_key example.key;
    }

Additionally, the ssl_reject_handshake directive makes configuring
certificates for the default server block optional.  If no certificates
are configured in the default server for a given listening socket,
certificates must be defined in all non-default server blocks with
the listening socket in question.

With the ssl_conf_command directive it is now possible to set
arbitrary OpenSSL configuration parameters as long as nginx is compiled
with OpenSSL 1.0.2 or later.  Full list of available configuration
commands can be found in the SSL_CONF_cmd manual page
(https://www.openssl.org/docs/man1.1.1/man3/SSL_CONF_cmd.html).

In particular, this allows configuring PrioritizeChaCha option
(ticket #1445):

    ssl_conf_command Options PrioritizeChaCha;

It can be also used to configure TLSv1.3 ciphers in OpenSSL,
which fails to configure them via the SSL_CTX_set_cipher_list()
interface (ticket #1529):

    ssl_conf_command Ciphersuites TLS_CHACHA20_POLY1305_SHA256;

Configuration commands are applied after nginx own configuration
for SSL, so they can be used to override anything set by nginx.
Note though that configuring OpenSSL directly with ssl_conf_command
might result in a behaviour nginx does not expect, and should be
done with care.

When enabled, certificate status is stored in cache and is used to validate
the certificate in future requests.

New directive ssl_ocsp_cache is added to configure the cache.

OCSP validation for client certificates is enabled by the "ssl_ocsp" directive.
OCSP responder can be optionally specified by "ssl_ocsp_responder".

When session is reused, peer chain is not available for validation.
If the verified chain contains certificates from the peer chain not available
at the server, validation will fail.

To evaluate variables, a request is created in the certificate callback,
and then freed.  To do this without side effects on the stub_status
counters and connection state, an additional function was introduced,
ngx_http_alloc_request().

Only works with OpenSSL 1.0.2+, since there is no SSL_CTX_set_cert_cb()
in older versions.

Early data AKA 0-RTT mode is enabled as long as "ssl_early_data on" is
specified in the configuration (default is off).

The $ssl_early_data variable evaluates to "1" if the SSL handshake
isn't yet completed, and can be used to set the Early-Data header as
per draft-ietf-httpbis-replay-04.