Fix interface name listing and resolve using snmp

I use this script for few years already and it is working fine!
I recently encounter problems when I monitored interface on Ubuntu 16. I checked and found the script used IF-MIB::ifDescr OID to look and query the interface name, which does not work for Ubuntu 16.
I guess it is a long runing bug or bad usage on this OID as there is a dedicated OID just for name: IF-MIB::ifName. after fixing it ubuntu 16 interfaces are monitored too so as the others that used to work with the previous OID. just replace the OID in the script.

FYI,
David