Break at_ifawithnet() into two variants:

Break at_ifawithnet() into two variants:

- at_ifawithnet(), which acquires an locks it needs and returns an
  at_ifaddr reference.
- at_ifawithnet_locked(), which relies on the caller locking
  at_ifaddr_list, and returns a pointer rather than a reference.

Update various consumers to prefer one or the other, including ether
and fddi output, to properly release at_ifaddr references.

Rework at_control() to manage locking and references in a manner
identical to in_control().

MFC after:	6 weeks