Capsicum: add capability mode to users binary

[tyler_tysdomain.com]

Summary

This patch adds capsicum support to the users binary.

Test Plan

Run users with no arguments, works well.

[cem]

usr.bin/users/users.cc

56

Does this need to include errno.h to compile?

[tyler_tysdomain.com]

Does this need to include errno.h to compile?

It compiles without, but you're right in that it should. The only reason it does compile is that some header somewhere also includes errno.h

[tyler_tysdomain.com]

1. Updating D9046: Capsicum: add capability mode to users binary #
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Updated to include extra err.h and errno.h

[oshogbo]

It's look good with me.
To be super precise we could consider limiting standard descriptors (std{in, out, err}).

[emaste]

In D9046#186953, @oshogbo wrote:

It's look good with me.
To be super precise we could consider limiting standard descriptors (std{in, out, err}).

This is a valid point, but I'm willing to trade off simplicity and smaller changes against precisely limiting stdio on trivial utilities like this.