Rewrite chuser() for simplicity and correctness

[klemensn]

• 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.

[neilalexander]

I'm OK with this, we might be able to write a unit test in core_test.go to prove that it does the right thing on Linux etc.

[klemensn]

I'm OK with this, we might be able to write a unit test in core_test.go to prove that it does the right thing on Linux etc.

I simply replaced the existing code.

Looking into tests now...

[klemensn]

Now with macOS building, still looks good on OpenBSD:

$ id nobody
uid=32767(nobody) gid=32767(nobody) groups=32767(nobody)
$ ps -c -U nobody -o command,user,ruser,svuid,group,rgroup,svgid,supgrp
COMMAND          USER     RUSER    SVUID GROUP    RGROUP   SVGID SUPGRP
yggdrasil        nobody   nobody   32767 nobody   nobody   32767 nobody

[klemensn]

I'm OK with this, we might be able to write a unit test in core_test.go to prove that it does the right thing on Linux etc.

test_chuser_unix.go testing chuser() alone is a start.

[klemensn]

Unless you run tests as root (who does that?), there is no way to test all of that code automatically.

@neilalexander I've abused an optional function parameter to cover the actual set*id(2) syscalls in CI.
It should work, but it is more of a PoC, which is to say, that I'd rather not have that chuser() change land.