typesafe callbacks

It's possible to portably make callbacks typesafe, as well as taking the 
existing void * arg.  This makes the callers more readable and more typesafe.

Done in a way that doesn't break non-gcc: they just don't get a warning.

Been in linux-next for about two full cycles now, and I really want to start 
using this in my own code.

Thanks,
Rusty.