vfork can cause memory corruption due to the lack of #[ffi_returns_twice]

[Amanieu]

Due to the lack of a stable #[ffi_returns_twice] attribute, vfork cannot be safely used since it can cause memory corruption of unrelated local variables in the parent process.

Miscompilation example: https://play.rust-lang.org/?version=nightly&mode=release&edition=2018&gist=e192ab81e8d408fa9984f766e2356697

Fixed (but nightly-only) version with #[ffi_returns_twice]: https://play.rust-lang.org/?version=nightly&mode=release&edition=2018&gist=0de019e56b69aa048f3e0f5319cc187e

[tgross35]

vfork has been deprecated #1574 then removed for 1.0 #3624, so this should be good to close.