Support for openat2()

[Serentty]

On Linux, the recent openat2() system call provides a broader API surface than openat(). However, it's not yet wrapped in GLIBC, so it would be necessary to make the system call directly. Would this be a problem in terms of maintainability and portability, or is this something that the Nix crate is already used to dealing with?

[asomers]

It's highly annoying, but not a deal-breaker. Nix already has a few functions that do that. Just grep Nix's codebase for libc::syscall.