Add the #starttls method for TCP sockets

[zeroSteiner]

This PR makes 3 changes, one in each commit, the most important of which adds the ability to initialize an SSL/TLS session with the peer when using a TCP socket on demand. This is particularly useful for protocols that implement SSL/TLS negotiation after some sort of exchange, e.g. SMTP, PostgreSQL, MSSQL, LDAP. With this method, the caller of the socket can implement the necessary protocol-specific exchange, then switch the socket to SSL/TLS on demand.

Extra changes include

• Extending socket pairs with the appropriate Rex::Socket class, which is needed so a TCP socket pair will have the new #starttls method
• Dropped old patches for Ruby versions older than 1.9.2 since they're no longer necessary (Metasploit uses 3.2+ these days).

Testing

There will be a Framework PR that'll require this and use the changes that can be used for testing. It'll be linked once it's open.