std: small `sys` refactor Part of rust-lang/rust#117276 The large number of files changed just results from the need to update a lot of imports. Actually this PR only: * combines the two definitions of `RawOsError` in `sys::pal` into one in `sys::io` * moves `FULL_BACKTRACE_DEFAULT` from `sys::pal` to `sys::backtrace` * moves the `FromInner`/`IntoInner`/... traits into `sys` (in preparation for removing `sys_common` entirely)