cmd/upspinfs: close the auth clients used to probe for cascheserver.
cmd/upspinfs: close the auth clients used to probe for cascheserver.

Also don't specify a keep alive interval since they won't last
long. Before this change upspinf would crash as soon as they
tried keep alives.

Change-Id: I54c9da9b0738a45bbed18d2da1c29c1dc3ae20ed
Reviewed-on: https://upspin-review.googlesource.com/6942
Reviewed-by: David Presotto <516b9783fca517eecbd1d064da2d165310b19759@google.com>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>

upspin/upspin,upspin/upspin