Auto merge of #149930 - joboet:small-sys-refactor, r=ChrisDenton

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)

Author: bors