RFC: io and os reform

This RFC proposes a significant redesign of the `std::io` and `std::os` modules
in preparation for API stabilization. The specific problems addressed by the
redesign are given in the Problems section below, and the key ideas of the
design are given in Vision for IO.

[Rendered](https://github.com/aturon/rfcs/blob/io-os-reform/text/0000-io-os-reform.md)
Most of the functionality is already portable with the exception of one variant of the enum explicitly marked as non-stable. It doesn't make sense to drop the entire thing because of one or two minor issues with the API that can be taken care of via extension traits.

> Differences like this show that this API is not quite ready for prime time just yet.

The section you're linking to claims that Windows doesn't have the concept of a primary user and group. Gaps in the understanding of the people behind this RFC indicate that they're not yet ready to be involved in designing any of this.