#52 Add support for SSL-based access to proxy servers

[Andrei Tarassov]

Trunk version of Apache allows configuring this web server as a tunneling proxy with SSL-based access:
https://issues.apache.org/bugzilla/show_bug.cgi?id=29744

In nutshell, one can connect to Apache via TCP/SSL and get the usual SSL security stuff - encryption and mutual certificate authentication - and then send the HTTP CONNECT command to have Apache tunnel the request to some other server. It would be good if libcurl (and curl command line tool as well) supported this kind of a setup. Namely, the SSL options which one can specify for a usual HTTPS connection should be also available for proxy connections, including:
* protocol (SSLv2, SSLv3, TLS, etc.)
* client certificate/key
* how server (proxy) certificate is to be verified (CA, verifying signature, verifying hostname match)

Note that one can connect to an HTTPS (or FTPS) server via an SSL-enabled tunneling proxy, thus there needs to be two sets of SSL-related settings: for connecting to the proxy and for connecting to the real target server.