include: Define IN6ADDR_ANY_INIT and IN6ADDR_LOOPBACK_INIT and code for IN6_IS_ADDR_LOOPBACK

[Jeff Latimer]

Jeff Latimer wrote:
> This patch enables putty to compile and run using IPv6.
I meant to say that André Hentschel was doing the hard work with putty 
and merely ask me to do this little bit.

Jeff