apparmor: convert from securityfs to apparmorfs for policy ns files

Virtualize the apparmor policy/ directory so that the current
namespace affects what part of policy is seen. To do this convert to
using apparmorfs for policy namespace files and setup a magic symlink
in the securityfs apparmor dir to access those files.

Signed-off-by: John Johansen <john.johansen@canonical.com>
Reviewed-by: Seth Arnold <seth.arnold@canonical.com>
Reviewed-by: Kees Cook <keescook@chromium.org>