Allow user to pass a pre-configured SSL_CTX to the client.

[pgit]

When using a secure connection, the onSecuring() callback is invoked with a pointer to a freshly created OpenSSL SSL object. You can then install certificates etc. on this session object.

This session object is created from a 'template' SSL_CTX object, which stores the defaults for all newly created SSL sessions. Some applications might have a pre-configured SSL_CTX and use that for configuring the SSL connection.

This patch adds a way to pass a pre-configured SSL_CTX to the connection, which is used instead of the one that is constructed internally.

[EmielBruijntjes]

Have you also tried to add extra methods to the TcpHandler class for initialization/construction of the ssl context? I find the extra parameter a bit odd.

[pgit]

Added a new overload of onSecuring() that userspace can use to return a SSL_CTX. Not quite sure about naming here. Also, are there no concerns about ABI breakage from adding a new method to the vtable?