Actually try making a userns before assuming they don't work

If unprivileged userns are *believed* to be disabled (such as with
"kernel.unprivileged_userns_clone = 0"), Lix would previously *give up*
on trying to use a user namespace before actually trying it, even if, in
cases such as unprivileged_userns_clone, it would actually be allowed
since Nix has CAP_SYS_ADMIN when running as daemon.

(see, e.g. https://github.com/archlinux/linux/commit/25d4709a4fc0e4f3b432c24b60dd508fb84f0cb2)

We changed it to actually try it first, and then diagnose possible
causes, and also to be more loud about the whole thing, using warnings
instead of debugs. These warnings will only print on the first build run
by the daemon, which is, tbh, eh, shrug.

This is what led to us realizing that no-userns was a poorly exercised
condition.

Change-Id: I8e4f21afc89c574020dc7e89a560cc740ce6573a

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