This abstracts away the details of particular input devices for netplay,
and adds support for mice and (similar) lightguns. Unfortunately, in
practice, no core handles mice or lightguns in a savestate-safe way, so
they need to be used in stateless mode to be reliable, but they do work.
The new input handling makes adding and removing players more
complicated, since data can be present that's not expected from the
connected clients list, or absent that's expected in the list but
actually shouldn't be there.
