(currently) only consumer (en).
Add a sysctl node hw.atm where the atm drivers will hook on their hardware
sysctl sub-trees.
Make atm_ifattach call if_attach and remove the corresponding call to if_attach
from en. Create atm_ifdetach and use that in en.
While the last change actually changes the interface this is not a problem in
practice because the only other consumer of this API is an older LANAI driver
on the net, that is not ready for current anyway.
Reviewed by: -atm
d2c96fc51f