Update if.h to match Linux kernel headers?

[Joseph Myers]

On Thu, 16 Jun 2016, Carlos O'Donell wrote:

> I'm not entirely sure if userspace can make use of these flags, but one
> could conceivably write userspace tools and drivers that would.

In <https://sourceware.org/ml/libc-alpha/2014-06/msg00413.html> I noted 
that I presumed the exclusion of IFF_* values not fitting in a "short" 
flags field was deliberate.

-- 
Joseph S. Myers
joseph@codesourcery.com