Rollup merge of #51809 - drrlvn:rw_exact_all_at, r=alexcrichton

Add read_exact_at and write_all_at methods to FileExt on unix

This PR adds `FileExt::read_exact_at()` and `FileExt::write_all_at()`, which are to `read_at()` and `write_at()` as `read_exact()` and `write_all()` are to `read()` and `write()`. This allows the user to not have to deal with `ErrorKind::Interrupted` and calling the functions in a loop.

I was unsure as to how to mark these new methods so I marked them `unstable`, please let me know if I should have done it differently.

I asked in Discord and was told that as this change is small it does not require an RFC.