s/unsafe/safe for all FFI imports

Hi,

I ran into some busy waiting in some of my own FFI bindings, which lead me to locate some possible busy waiting and concurrency issues in these bindings.

As per:
http://blog.ezyang.com/2010/07/safety-first-ffi-and-threading/

Is there any reason these shouldn't all be safe, so that the runtime can make these calls concurrent?