Documents the safety invariants for the AtomicPtr<()> that caches
the memory scheme root fd for the process lifetime:
- read-only after first init (Ordering::Acquire)
- pointer is either null or a valid fd cast to *mut ()
- cast is valid because we never dereference the pointer; only
round-trip through libredox::call::dup
This is the first of multiple commits adding # Safety documentation
across the unsafe surfaces of redox-driver-sys.
vasilito
0a559c2e18