Distinguish SNMP failures from absent objects in genpdu

A missing object is exception text under v2c/v3 and an error under v1,
and genpdu treated both the same. Add pdu2_get, returning a value and an
ok, absent or failed status. A failed signed minimum read now skips the
sensor, and rinv no longer prints "No Such Instance currently exists at
this OID" for a nameplate field the model does not populate.

Read the signed minimum per entity: the MIB indexes it that way, and it
decides which value column is read.

In the session probe, fall back to the PDU2 nameplate rather than
sysDescr, so a credential that cannot read PDU2 is rejected, return
early when the pduCount read fails at the transport level, and warn once
per PDU.

Signed-off-by: Kilian Cavalotti <kilian@stanford.edu>