viced: Enable NAT ping on hosts

Turn on NAT ping on the Rx connection for the callback channel for
hosts. This should help improve behavior for clients behind NATs and
stateful firewalls, even for clients that predate NAT ping
functionality.

Reviewed-on: http://gerrit.openafs.org/4646
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit aafdc08cfc49da4c23ecd91f9e690fd70e95df55)

Change-Id: I428b6648276ec49fd4f003b3cf2d88a07c8aa1d9
Reviewed-on: http://gerrit.openafs.org/9420
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>