[FEATURE REQUEST] adjust `unix_add_fd` and friends to use `BorrowedFd` instead of `RawFd`

[nagisa]

This represents the correct ownership non-transfer more clearly and allows users to avoid unsafe in their code when trying to use the file descriptor in any way.

[sdroege]

See #871

[bilelmoussaoui]

This can be closed, any missing items can be handled separately