libc: Shortcut if_indextoname() if index == 0

[kp]

Summary

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
Sponsed by: RG Nets
MFC after: 2 weeks

[donner]

Is there really a typical use case for this call?

[kp]

Yeah, pfctl calls this, and there are flows where it gets called with an index of 0. There's no point in asking the kernel, that only takes time and doesn't change the answer.
We could take this shortcut in pfctl, but it makes more sense to do add the shortcut for everyone.