mod_ssl: negotiate the TLS protocol version per name based vhost configuration.

mod_ssl: negotiate the TLS protocol version per name based vhost configuration.

By using the new ClientHello callback provided by OpenSSL 1.1.1, which runs at
the earliest connection stage, we can switch the SSL_CTX of the SSL connection
early enough for OpenSSL to take into account the protocol configuration of the
vhost.

In other words:
    SSL_set_SSL_CTX(c->SSL, s->SSL_CTX)
followed by:
    SSL_set_{min,max}_proto_version(SSL_CTX_get_{min,max}_proto_version(s->SSL_CTX))
works as expected at this stage (while the same from the SNI callback is
ignored by/due to OpenSSL's state machine).

Extracting the SNI (to select the relevant vhost) in the ClientHello callback
is not as easy as calling SSL_get_servername() though, we have to work with
the raw TLS extensions helpers provided by OpenSSL. I stole this code from a
test in the OpenSSL source code (i.e. client_hello_select_server_ctx() in
test/handshake_helper.c).

We can then call init_vhost() as with the SNI callback (in use only for OpenSSL
versions earlier than 1.1.1 now), and pass it the extracted SNI.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1868645 13f79535-47bb-0310-9956-ffa450edef68