ip6_addr_assign_zone macro

Assign a zone index to an IPv6 address, based on a network interface. If the given address has a scope, the assigned zone index is that scope's zone of the given netif; otherwise, the assigned zone index is "no zone". This default implementation follows the default model of RFC 4007, where only interface-local and link-local scopes are defined, and the zone index of both of those scopes always equals the index of the network interface. As such, this default implementation need not distinguish between different constrained scopes when assigning the zone.