snmp interface oids

[Jeff Weisberg]

I've added a new feature to help with SNMP. any oid
which has the form "blah.blah.blah.interface_number"
(such as anything in the 'if' table) can now be specified
using the interface name:

        oid:  ifOperStatus[Serial1/0]
        oid:  ifInOctets[FastEthernet0/1]
        oid:  isdnLapdOperStatus[Serial4/2:23]

argus will query the device and find the interface
number that matches the name. and argus will periodically
check and update the oid if it changes.