FreeBSD and NetBSD don't have secure_getenv(3), same as macOS.
FreeBSD 13 and lower also don't allow setting environ pointers to null.
Co-Authored-By: Robert Clausecker <fuz@FreeBSD.org>
Core::System already had some wrappers for *env() functions, which I've
copied over. But 1) the set of functions there was incomplete, and 2)
the environment feels like an object in its own right, so let's treat it
as one. :^)
Also add `Core::Environment::has(StringView)` for situations where we
just care if a variable is defined.
