Previously, "%uA" was used, which corresponds to ngx_atomic_uint_t.
Size of ngx_atomic_uint_t can be easily different from uint64_t,
leading to undefined results.

In particular, it was not possible to obtain SSLv2 protocol version.

In TLSv1.3, NewSessionTicket messages arrive after the handshake and
can come at any time.  Therefore we use a callback to save the session
when we know about it.  This approach works for < TLSv1.3 as well.
The callback function is set once per location on merge phase.

Since SSL_get_session() in BoringSSL returns an unresumable session for
TLSv1.3, peer save_session() methods have been updated as well to use a
session supplied within the callback.  To preserve API, the session is
cached in c->ssl->session.  It is preferably accessed in save_session()
methods by ngx_ssl_get_session() and ngx_ssl_get0_session() wrappers.

The SSL_OP_NO_RENEGOTIATION option is available in OpenSSL 1.1.0h+ and can
save some CPU cycles on renegotiation attempts.

In OpenSSL 1.1.0 the SSL_CTRL_CLEAR_OPTIONS macro was removed, so
conditional compilation test on it results in SSL_clear_options()
and SSL_CTX_clear_options() not being used.  Notably, this caused
"ssl_prefer_server_ciphers off" to not work in SNI-based virtual
servers if server preference was switched on in the default server.

It looks like the only possible fix is to test OPENSSL_VERSION_NUMBER
explicitly.

Starting with OpenSSL 1.1.0, SSL_R_UNSUPPORTED_PROTOCOL instead of
SSL_R_UNKNOWN_PROTOCOL is reported when a protocol is disabled via
an SSL_OP_NO_* option.

Additionally, SSL_R_VERSION_TOO_LOW is reported when using MinProtocol
or when seclevel checks (as set by @SECLEVEL=n in the cipher string)
rejects a protocol, and this is what happens with SSLv3 and @SECLEVEL=1,
which is the default.

There is also the SSL_R_VERSION_TOO_HIGH error code, but it looks like
it is not possible to trigger it.

There should be at least one worker connection for each listening socket,
plus an additional connection for channel between worker and master,
or starting worker processes will fail.

Previously, listenings sockets were not cloned if the worker_processes
directive was specified after "listen ... reuseport".

This also simplifies upcoming configuration check on the number
of worker connections, as it needs to know the number of listening
sockets before cloning.

The variable keeps the latest SSL protocol version supported by the client.
The variable has the same format as $ssl_protocol.

The version is read from the client_version field of ClientHello.  If the
supported_versions extension is present in the ClientHello, then the version
is set to TLSv1.3.