Adds a new libretro interface for a core to send and receive custom network packets for implementing a communication based multiplayer system instead of using the default state serialization based multiplayer. Connection management is still done by the frontend while a core gains the ability to easily support tunneling of multi-console data communication traffic.
* Rewind during recording isn't visibly busted anymore but it doesn't rewind the replay properly during playback or record, inputs get clobbered; check frame pos/ptr stuff.
* Fix rewinding during movie recording and playback?
* Disable run-ahead based on save state support level defined in core info files
* Disable rewind based on save state support level defined in core info files