Rollup merge of #40842 - Mic92:RawFd, r=aturon

Implement AsRawFd/IntoRawFd for RawFd

This is useful to build os abstraction like the nix crate does.
It allows to define functions, which accepts generic arguments
of data structures convertible to RawFd, including RawFd itself.
For example:

```
fn write<FD: AsRawFd>(fd: FD, buf: &[u8]) -> Result<usize>
write(file, buf);
```
instead of:
```
fn write(fd: RawFd, buf: &[u8]) -> Result<usize>
write(file.as_raw_fd(), buf);
```

cc @kamalmarhubi