Instead of adding model and serial number to the graph itself, use a
static title for it and instead provide the model and serial number as
part of graph_info.
This change makes it much easier to forward the warning/critical
statuses back to Icinga (or Nagios), where you have to match the graph
title.
At least Munin 2.0 already provides snmp__printer_supplies that takes
care of the same data (they all use Printer-MIB values). Also that
plugin uses Munin's own SNMP abstraction instead of using net-snmp
commands directly, which makes it more suitable for the job.
