RFC 2533 refers to 'A Syntax for Describing Media Feature Sets',
which is wrong since the correct reference should be
RFC 2553 'Basic Socket Interface Extensions for IPv6'.
Obtained from: OpenBSD
MFC after: 1 week
If the index we're trying to convert is 0 we can avoid a potentially
expensive call to getifaddrs(). No interface has an ifindex of zero, so
we can handle this as an error: set the errno to ENXIO and return NULL.
Submitted by: Nick Rogers
Reviewed by: lutz at donnerhacke.de
MFC after: 2 weeks
Sponsored by: RG Nets
Differential Revision: https://reviews.freebsd.org/D24524
