Use an interface index instead of a pointer for multicast options.

Use an interface index instead of a pointer for multicast options.

Output interface (port) selection for multicast traffic is not done via
route lookups.  Instead the output ifp is registred when setsockopt(2)
is called with the IP{V6,}_MULTICAST_IF option.  But since there is no
mechanism to invalidate such pointer stored in a pcb when an interface
is destroyed/removed, it might lead your kernel to fault.

Prevent a fault upon resume reported by frantisek holop, thanks!

ok mikeb@, claudio@