- Use unambiguous variable names (w/o package name conflict).
- Fail on invalid input such as the empty string or `:`.
- Do not change group without user, i.e. fail on `:group`.
- Parse input using mnemonic APIs.
- Do not juggle between integer types.
- Unset supplementary groups.
- Use set[ug]id(2) to follow the idiom of OpenBSD base programs.
(cannot use setres[ug]id(2) as macOS does not have them.)
Includes/Supersedes #1202.
Fixes#927.
I only tested on OpenBSD (so far), but other systems should just work.
