Updates to SNMP sync code for Bugs #933, #939 & #941, plus Cisco 2921

Also moved test of OID against $known_switches to stop the redundant query of SNMP sysName & SysDescr which are not used if the OID is not in the known_switches array.

2 new devices added to dictionary.

1 iftable_processor removed as it replicated another processor in all but the name.

1 iftable_processor split to allow for new device configuration