Auto merge of #25125 - Stebalien:from_raw_os, r=alexcrichton

So, I realize this is really late in the game so it's unlikely to be accepted but `FromRawFd`/`FromRawHandle` are necessary for fine grain control over file creation. For example, the current `OpenOptions` does not provide a way to avoid file creation races (there's no way to specify `O_EXCL` or the windows equivalent). Stabilizing these traits and their implementations will give 1.0 users fine-grain control over file creation without committing to any new complex APIs.  Additionally, `AsRawFd`/`AsRawHandle` are already stable so I feel that that stabilizing their inverses is a reasonably small change.

Disclaimer: I'm asking because my crate, tempfile, depends on this feature.