windows/netinet: Copy ip6.h and icmp6.h from netbsd.

windows/netinet: Copy ip6.h and icmp6.h from netbsd.

There are a few structure definitions that is used from
these headers. So copy them from the netbsd repo.

The following changes have been made on top of it:
* The keyword "__packed" has been removed
from the headers as the corresponding Linux headers don't
do packing.
* #if BYTE_ORDER == 'X' macros have been replaced by CONSTANT_HTONx().
* code inside #ifdef _KERNEL has been deleted.
* code inside #ifdef ICMP6_STRINGS has been deleted.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>