Summary:
We still have naked pointers at the handoff to the kernel, but now
have a cleaner implementation at the level that we're going to be working at
day to day.
I also renamed `FileHandle::release` to `FileHandle::releasefile` so that it
isn't visually ambiguous with `std::unique_ptr::release` in the
`Dispatcher.cpp` code: `fh.release()` vs `fh->release()` look similar but are
dramatically different in behavior.
Reviewed By: bolinfest
Differential Revision: D3309455
fbshipit-source-id: f8cf055bcd51121048a20f0202988cf0aef1f085
