Update if.h to match Linux kernel headers?

[Szabolcs Nagy]

On 17/06/16 10:12, Joseph Myers wrote:
> 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.
> 

musl has these for completeness anyway, the
flags can be queried from userspace via
/sys/class/net/<dev>/flags
(i also added IFF_VOLATILE to musl if.h)