Windows: rx lowlevel nat ping

Windows: rx lowlevel nat ping

sha1 d24078658d183ea2e72e61c1888e9900bac0ec32 added support
for a low-level nat ping using rx version replies.

Export rx_SetConnSecondsUntilNatPing from afsrpc.dll.

Add NatPingInterval registry value to
  HKLM\SYSTEM\CurrentControlSet\Services\TransarcAFSDaemon\Parameters
to permit Nat Ping to be enabled.  The default value is 0 seconds.

Change-Id: I5122ea81e1a61934eba3c836632039c76f6c45e3
Reviewed-on: http://gerrit.openafs.org/1487
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>