Fix in6_multi double free

Fix in6_multi double free

This is actually several different bugs:
- The code is not designed to handle inpcb deletion after interface deletion
  - add reference for inpcb membership
- The multicast address has to be removed from interface lists when the refcount
  goes to zero OR when the interface goes away
  - decouple list disconnect from refcount (v6 only for now)
- ifmultiaddr can exist past being on interface lists
  - add flag for tracking whether or not it's enqueued
- deferring freeing moptions makes the incpb cleanup code simpler but opens the
  door wider still to races
  - call inp_gcmoptions synchronously after dropping the the inpcb lock

Fundamentally multicast needs a rewrite - but keep applying band-aids for now.

Tested by: kp
Reported by: novel, kp, lwhsu