IOS: Rewrite FS to use FileSystem

This is the large change in the branch.

This lets us use either the host filesystem or (in the future) a NAND
image exactly the same way, and make sure the IPC emulation code
behaves identically. Less duplicated code.

Note that "FileIO" and "FS" were merged, because it actually doesn't
make a lot of sense to split them: IOS handles requests for both
/dev/fs and files in the same resource manager, and as it turns out,
/dev/fs commands can *also* be sent to non /dev/fs file descriptors!
If we kept /dev/fs and files split, there would be no way to
emulate that correctly. I'm not aware of anything that does that (yet?)
but I think it's important to be correct.

Source/Core/Common/Logging/Log.h

@@ -32,6 +32,7 @@ enum LOG_TYPE
   IOS_DI,
   IOS_ES,
   IOS_FILEIO,
+  IOS_FS,
   IOS_NET,
   IOS_SD,
   IOS_SSL,