avoid nat ping for authenticated connections

avoid nat ping for authenticated connections

this is a cheap way to ensure one nat ping per server. basically,
only do it for the one unauth connection we have.

Change-Id: I4583112d7b4a10a0e8f524795527d99a5b2adf51
Reviewed-on: http://gerrit.openafs.org/2010
Reviewed-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>