rrdtool1: modify logarithmic scale labelling

rrdtool1: modify logarithmic scale labelling

The y-axis graph labels in logarithmic mode display in 'scientific'
notation eg: '1e+00' for 0, '1e+01' for 10, '1e+02' for 100 and so on.
This IMO is a pain in the backside for non scientific humans to read.

Modified output to display numbers up to 99,999 in conventional decimal
format and to revert to scientific notation for larger, thus the same
display space is taken.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
(cherry picked from commit edc41b26ab1e78d55e87c79562d8430b87d314a7)