rx lowlevel nat ping

rx lowlevel nat ping

for rfc 4787, do a minimal impact nat ping. this uses an rx
"version request" reply debug packet, which will simply be discarded by the
receiver, to keep the port mapping open.

Change-Id: Ic2180bfa5c467e33c72e3f19d62488bd6a2dc61a
Reviewed-on: http://gerrit.openafs.org/1393
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>