SNMP: provide more precise VCC, temp, RX/TX power from SFPs
Temperature is reported by SFP in units of 1/256 C, SNMP exports it with the resolution 1 C. VCC is reported by SFP in 0.1mV, SNMP exports it with the resolution 1 mV. RX/TX power is reported by SFP in 0.1mW, SNMP exports it with the resolution 1 mW.
Add new OIDs to support more precise reporting.