Graph the max value rather than avg if rrdsingle disabled

With rrdsingle the average values are used for graphing. As the
timescale increases (say over a day or month) the average values
progressively reduce.  Using the maximum value over the longer periods
is arguably no less deceptive but it does produce more informative
graphs.

V2 - Invert the default path logic related to rra.single

Signed-off-by: Kevin Darbyshire-Bryant <kevin@darbyshire-bryant.me.uk>