MFC 253751 and 253843:

- Relax the restriction on the member interfaces with LLAs.  Two or more
  LLAs on the member interfaces are actually harmless when the parent
  interface does not have a LLA.

- Add net.link.bridge.allow_llz_overlap.  This is a knob to allow LLAs on
  a bridge and the member interfaces at the same time.  The default is 0.

Approved by:	re (marius)