Generic check for printers supporting SNMP Printer MIB

This Nagios plugin queries prtAlertTable from the Printer MIB.

It honors only prtAlertTrainingLevel = trained(4) or fieldService(5),prtAlertTrainingLevel = other(1),unknown(2),untrained(3) and management(6) are ignored.

For prtAlertSeverityLevel = warning(4) the plugin result is "WARNING", 
for critical(3) the plugin result is "CRITICAL", obviously,
for warningBinaryChangeEvent(5) (new in RFC3805) the state is by default
"WARNING", but there is a hardcoded table of change events, which result in
state "CRITICAL".