Add safe Rust wrappers around the raw userfaultfd ioctls: create
(syscall + API handshake), register (missing-page mode), copy
(resolve fault), and wake (unblock threads after EEXIST race).
These are used by the demand-paged snapshot restore handler in a
subsequent commit.
Signed-off-by: Shayon Mukherjee <shayonj@gmail.com>
