Add FreeBSD's Capsicum constants

[asomers]

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[asomers]

The CI failures are unrelated.

[asomers]

Note that the CAP_RIGHTS_VERSION constant violates the rule against unstable constants. It's a version number used to tell the kernel which version of the other CAP_* rights the program is using. So I think it's appropriate to include it here. We must increment it in the future if there's a ever a new version of the rights.