redox: add get/setresgid and get/setresuid

[willnode]

Description

This defines 4 functions in unistd.h for redox daemon system to work:

• getresgid
• getresuid
• setresgid
• setresuid

These functions have been tested in C ports that uses them, such as nginx.

Sources

• getresgid
• getresuid
• setresgid
• setresuid

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

@rustbot label +stable-nominated