Disable some stuff in IPC_HLE when determinism is required.

This includes net, which is nonsensical to "sync" in netplay/replays
(could have the host do the Wii networking in future though...), and USB
keyboard, which just needs some love to do the same.
Add a mode to use a dummy Wii NAND.

Eventually, netplay will be able to use the host's NAND, but this could
still be useful in some cases; for TAS it definitely makes sense to have
a way to avoid using any preexisting NAND.