Get WebRTC backend working again

Poking at the test `sockets.test_webrtc`, I see that we have removed the whole WebRTC backend from Emscripten altogether. WebRTC used to be implemented in `src/library.js`, see e.g. at this point of time: https://github.com/kripken/emscripten/blob/3b99bb7bef220ad98760b117d14854098f86db9e/src/library.js#L7032, where the two implementations WebRTC vs WebSockets were chosen based on a `#if SOCKET_WEBRTC`. Looks like the WebRTC part of it was never integrated as part of the filesystem in `src/library_sockfs.js`, which only carries WebSockets -specific code.

This commit https://github.com/kripken/emscripten/commit/d92efe09c6bc825539f9ce8f7e10ac2fcde0bdaf removes the sockets specific code from `src/library.js`, but looks like the WebRTC implementation was not migrated anywhere else, so it got lost as part of the refactoring.