fill_autodev: bind-mount if mknod fails (v3)

fill_autodev: bind-mount if mknod fails (v3)

First, rename setup_autodev to fill_autodev, since all it
does is populate it, not fully set it up.

Secondly, if mknod of a device fails, then try bind-mounting
it from the host rather than failing immediately.

Note that this isn't an urgent patch because the common.userns
configuration hook already specifies bind,create=file mount
entries for all the devices we would want.

Changelog (v3): ignore if /dev doesn't exist

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>