Add prepareSecure method to tcpsocket

Add prepareSecure method to tcpsocket, to fix https://github.com/uProxy/uproxy/issues/413. In freedom-for-chrome, this will call .setPaused, but in freedom-for-firefox and node this will be a no-op
This bug isn't a great explanation for someone who wants to work with sockets. Can we have a freedom wiki page on what the semantics are to correctly secure a socket, along with a plan for when we can get rid of prepareSecure instead?