Auto merge of #1574 - Amanieu:deprecate-vfork, r=gnzlbg

Deprecate vfork

The compiler may generate incorrect code for `vfork` and `setjmp` because they are missing the `#[returns_twice]` attribute which is currently unstable ([tracking issue](https://github.com/rust-lang/rust/issues/58314)). Since `vfork` is impossible to use safely, I propose deprecating it until `#[returns_twice]` is stable.