generic/317,318: fail gracefully if userns not supported

generic/317,318: fail gracefully if userns not supported

generic/317 and generic/318 fail un-gracefully on older kernels
which don't support userns; fix that by running a simple test
as a prerequisite and fail gracefully if needed.

Roll that in with the test for executable presence, and make
a new _require_userns()

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>