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For some reason the RHEL6 kernel-headers package doesn't have it.

Let's just follow for now the cargo culting of "define defines ourself
if not available" that various kernel-tied utilities have because
various buildsystems are too shitty to make it easy to install newer
kernel headers even if you're running an old kernel.

It was just an extra check to be sure we would be switching back to
the right uid, but there's no reason not to allow executing this
program as root.

Just more convenient, and there's no reason not to.

This is convenient for when we just want e.g. --unshare-net.