[krbdev.mit.edu #7960] IPv6 pktinfo not used in Linux build

[Greg Hudson via RT]

For reasons not yet clear to me, a Linux build environment (e.g. Ubuntu 
12.04 or 14.04):

* declares struct in6_pktinfo in <netinet/in.h> only if __USE_GNU is 
defined, which is only the case if _GNU_SOURCE is defined by the 
compiled code.

* declares struct ip_pktinfo in <bits/in.h> (included by netinet/in.h) 
if either __USE_GNU or __USE_MISC is defined.  The latter is true by 
default because _BSD_SOURCE is defined by default (if no other feature 
macros are used).

This makes little sense to be, since the IPv6 pktinfo API is specified 
in RFC 3542, while ip_pktinfo is less standardized and I believe 
originated in Linux.

The upshot is that we are not using IPv6 pktinfo on Linux, and instead 
are creating one UDP listener socket per non-loopback IPv6 interface 
address.

Combined with issue #7526, this adds more motivation to use 
AC_USE_SYSTEM_EXTENSIONS and deal with any consequent damage (such as 
the different behavior of strerror_r).  But there are more conservative 
workarounds, such as moving the pktinfo code to a separate file and 
defining _GNU_SOURCE in that file.