Change SocksToRtc to have a freedom-compatible interface.

This change also adds a manifest.json for SocksToRtc, but this
manifest is not yet used anywhere or verified by the build.
We spent a bunch of time today talking about this.  I think it's a good idea, and we should try something along those lines, but it looks sufficiently far from trivial that it's better off in a separate pull request.