Auto merge of #26159 - alexcrichton:tweak-process-lowering-raising, r=brson

Auto merge of #26159 - alexcrichton:tweak-process-lowering-raising, r=brson

* Slate these features to be stable in 1.2 instead of 1.1 (not being backported)
* Have the `FromRawFd` implementations follow the contract of the `FromRawFd`
  trait by taking ownership of the primitive specified.
* Refactor the implementations slightly to remove the `unreachable!` blocks as
  well as separating the stdio representation of `std::process` from
  `std::sys::process`.

cc #25494