ip6_addr_has_scope macro

Determine whether an IPv6 address has a constrained scope, and as such is meaningful only if accompanied by a zone index to identify the scope's zone. The given address type may be used to eliminate at compile time certain checks that will evaluate to false at run time anyway. This default implementation follows the default model of RFC 4007, where only interface-local and link-local scopes are defined. Even though the unicast loopback address does have an implied link-local scope, in this implementation it does not have an explicitly assigned zone index. As such it should not be tested for in this macro.