AgentX index_allocate is unusable

[jimdigriz]

Net-SNMP's snmpd does not seem to provide a usable index allocation response.

Though the index allocation works as you would expect, the Register command seems unwilling to return duplicateRegistration making it impossible to use the process described in RFC 2741, section 7.1.4.2.2 covering Index Allocation; I am using the default priority (127) which looks to match what the source code uses for the internal interface interface entries.

This means you are unable to reserve an ifIndex safely for use in IF-MIB::ifTable

For now the only safe(r) strategy is to pick a high non-conflicting number and hope it pans out okay.