Log Message:
Updated net/mrtg to 2.15.1

* make cfgmaker detect broken snmpv1 counters more reliably
* latest Net_SNMP_util.pm with many small fixes
* more cfgmaker smarts and documentation for snmpv3
* be happy even if no threshmail sending is configured
* add install target for traffic-summer
* fix mrtg-traffic-sum default catch expression to be in sync with docs