MSRV policy for libc crate

In a previous libs team meeting, we discussed MSRV policy for the libc crate, and we debated between an N-1 policy and a slightly more conservative policy such as N-3 or N-4. We ended the meeting without having settled on a specific consensus, though it seemed like we were roughly on the same page.

Rather than make ad-hoc changes based on a non-specific MSRV policy, I think we should go ahead and nail down an MSRV policy for present and future changes.

I'd like to propose the following concrete policy:

• Support the last three stable versions of rustc (stable and the prior two versions).
• Only bump MSRV when we use a feature requiring newer Rust. (We do not systematically bump MSRV each time a stable release occurs.)
• Treat MSRV increases as an ordinary change, not a breaking change.

Note that this is a policy proposal for libc, not for other crates maintained by the libs team. After we get consensus on an MSRV policy for libc, we can consider what policy we want for other crates. Nonetheless, libc MSRV policy seems likely to serve as a defacto lower-bound for supported Rust versions among other libs team crates, and among many crates in the ecosystem.

For further information on the benefits of this: the PR dropping support and version detection for older versions of rustc was able to remove ~1600 lines of code, and that's not counting subsequent work that may be able to drop some of the wrapper macros. This represents a substantial improvement to maintainability, and may make it easier in the future to move to more automation to keep libc updated with the latest platform APIs.

Start by reading the proposed MSRV policy and the linked discussion, including the unresolved debate over N-1 versus N-3 or N-4 support. Done means the libs team reaches consensus on a libc policy and records the agreed rules for future changes.