Don't explicitly invoke ifn_start on new child interfaces (vaps and
subinterfaces) after they are created.  Interfaces are already started
by devd invoking /etc/pccard_ether when they are created, so the explicit
calls in childif_create() resulted in interfaces being started twice.

Note that interfaces created via cloned_interfaces are not explicitly
started but depend on the devd mechanism already.

MFC after:	1 week