statistics: support rrdtool's alt_autoscale and alt_autoscale_max options

statistics: support rrdtool's alt_autoscale and alt_autoscale_max options

Implement support for alternative scaling of the y-axis.
By default, rrdtool will autoscale to 1,2,5,10,20,50,100,200,... etc.,
which is not always suitable (e.g. memory charts for device with 128 MB).

Rrdtool 1.0.50 already supports alternative autoscaling that creates
a tighter y-axis. Implement graph-level options in Luci statistics to
support those boolean options as "alt_autoscale" and "alt_autoscale_max".

info at http://oss.oetiker.ch/rrdtool/doc/rrdgraph.en.html

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>