Fix auto-uid-allocation in Docker containers

This didn't work because sandboxing doesn't work in Docker. However,
the sandboxing check is done lazily - after clone(CLONE_NEWNS) fails,
we retry with sandboxing disabled. But at that point, we've already
done UID allocation under the assumption that user namespaces are
enabled.

So let's get rid of the "goto fallback" logic and just detect early
whether user / mount namespaces are enabled.

This commit also gets rid of a compatibility hack for some ancient
Linux kernels (<2.13).

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