Useful code added while debugging WHQL issues:

- Tidied up receiver xenstore feature advertisement
- Added registry options to turn off GSO at the transmitter
- Added an 'always copy' option to the transmtter
- Added AH option parsing for IPv6
- Added code to drop oversize non-GSO packets at receiver
- Added some more error DbgPrints
- Added large packet stats to the receiver

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>