pmwebd graphite webapi: correct filtering for non-normal floating-point data
pmwebd graphite webapi: correct filtering for non-normal floating-point data

In one context that is used by grafana json queries, we were mapping
only some NaN values to 'null', rather than all non-normal FP values.
This could result in outright JSON syntax errors, if literals like
'inf' were printed instead.  We now are more consistent with the
proper filter (isnormal(3)).

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp