Implement the extension api for sendfile to allow for kqueue notifications.

This is still under a bit of flux, as the final API hasn't been nailed
down. It's also unclear whether we should define the two new types in the
header or not - it may allow bad code to compile that shouldn't (ie,
since uintX's are defined, the developer may not include sys/types.h.)

Reviewed by: peter, imp, bde
Sponsored by: Netflix, Inc.