Fix the IPV6_MULTICAST_IF sockopt handling. RFC 3493 says when the
interface index is specified as zero, the system should select the
interface to use for outgoing multicast packets. Even the comment
for the in6p_set_multicast_if() function says about index of zero.
But in fact for zero index the function just returns EADDRNOTAVAIL.

I.e. if you first set some interface and then will try reset it
with zero ifindex, you will get EADDRNOTAVAIL.

Reset im6o_multicast_ifp to NULL when interface index specified as
zero. Also return EINVAL in case when ifnet_byindex() returns NULL.
This will be the same behaviour as when ifindex is bigger than
V_if_index. And return EADDRNOTAVAIL only when interface is not
multicast capable.

Reported by: Olivier Cochard-Labbé
MFC after: 2 weeks
Sponsored by: Yandex LLC