Relax the list of interfaces which support IPv6 a bit.

Relax the list of interfaces which support IPv6 a bit.

No longer require IFF_MULTICAST for all interfaces. It is save to skip
this for interfaces that don't require a ND cache. Also do not assign a
link-local address in such cases. This affects point-to-point interfaces
and the NBMA / point-to-multipoint interfaces like mpe(4), mgre(4) and wg(4).
The NBMA interfaces need some alternative way to figure out the address
mapping. In the end this allows non-multicast interfaces to work with IPv6.
OK dlg@ kn@