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>

Originally, the stream module was developed based on the mail module,
following the existing style.  Then it was diverged to closely follow
the http module development.  This change updates style to use sscf
naming convention troughout the stream module, which matches the http
module code style.  No functional changes.

Server name is taken either from ngx_stream_ssl_module or
ngx_stream_ssl_preread_module.

The change adds "default_server" parameter to the "listen" directive,
as well as the following directives: "server_names_hash_max_size",
"server_names_hash_bucket_size", "server_name" and "ssl_reject_handshake".

The directive sets the server list of supported application protocols
and requires one of this protocols to be negotiated if client is using
ALPN.

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.

In mail and stream modules, no certificate provided is a fatal condition,
much like with the "ssl" and "starttls" directives.

In http, "listen ... ssl" can be used in a non-default server without
certificates as long as there is a certificate in the default one, so
missing certificate is only fatal for default servers.

New directives: "ssl_verify_client", "ssl_verify_depth",
"ssl_client_certificate", "ssl_trusted_certificate", and
"ssl_crl".

New variables: $ssl_client_cert, $ssl_client_raw_cert,
$ssl_client_s_dn, $ssl_client_i_dn, $ssl_client_serial,
$ssl_client_fingerprint, $ssl_client_verify, $ssl_client_v_start,
$ssl_client_v_end, and $ssl_client_v_remain.