nspawn: check if kernel supports userns as early as possible

nspawn: check if kernel supports userns as early as possible

If the kernel do not support user namespace then one of the children
created by nspawn parent will fail at clone(CLONE_NEWUSER) with the
generic error EINVAL and without logging the error. At the same time
the parent may also try to setup the user namespace and will fail with
another error.

To improve this, check if the kernel supports user namespace as early
as possible.