afsmonitor: add fs callback xstats collection

afsmonitor: add fs callback xstats collection

Adds the capability to afsmonitor to display and process
thresholds for fileserver callback xstats collections. The
callback counters are placed to the right of the full perf
data.  When afsmonitor is started without a configuration
file, the callback xstats data is retrieved and displayed,
since by default afsmonitor will show every field.  When a
configuration file is given, poll the fileserver only for the
collections needed to display information requested by the
'show fs' commands.

Change-Id: I572ff682de4cc7ef27bb46dd028d3d797b873841
Reviewed-on: http://gerrit.openafs.org/1731
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>