std: small sys refactor

[joboet]

Part of #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)

[rustbot]

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