redox: Enable getres/setres

[willnode]

What does this PR do

This enables getset/setres unistd API for Redox OS. This has been added to libc since rust-lang/libc#4752, and tested for months by porting rustysd to the platform.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[SteveLauC]

Let me rebase your branch to resolve the CI failures.

[SteveLauC]

There 2 tests should be enabled for redox as well. Since this is a trivial change, let me do it and merge the PR.

nix/test/test_unistd.rs

Lines 724 to 740 in 759c65c

	#[cfg(any(linux_android, freebsdlike, target_os = "openbsd"))]
	#[test]
	fn test_getresuid() {
	let resuids = getresuid().unwrap();
	assert_ne!(resuids.real.as_raw(), libc::uid_t::MAX);
	assert_ne!(resuids.effective.as_raw(), libc::uid_t::MAX);
	assert_ne!(resuids.saved.as_raw(), libc::uid_t::MAX);
	}
	#[cfg(any(linux_android, freebsdlike, target_os = "openbsd"))]
	#[test]
	fn test_getresgid() {
	let resgids = getresgid().unwrap();
	assert_ne!(resgids.real.as_raw(), libc::gid_t::MAX);
	assert_ne!(resgids.effective.as_raw(), libc::gid_t::MAX);
	assert_ne!(resgids.saved.as_raw(), libc::gid_t::MAX);
	}

[SteveLauC]

Thanks!