Add genpdu pdutype for generic SNMP PDUs (PDU2-MIB)

Add a new pdutype (genpdu) for PDUs implementing the Raritan PDU2-MIB.
A single MIB covers the Raritan PX2/PX3/PX4/PXC/SRC/PXO/BCM series, the
Server Technology PRO3X/PRO4X series, the Legrand intelligent PDUs, and
all PDUs following the PDU2-MIB.

Supports rpower (whole-PDU and per-outlet), rinv and rvitals over SNMP
v1, v2c or v3, with credentials read from the pdu table).

Unlike the existing types, sensor units and decimal precision are read
from the MIB per sensor rather than hardcoded, so readings are correct
across models that report differing precision for the same sensor.
Outlet switching capability is probed at connect time, so metered-only
models report a single "unsupported" message instead of a per-outlet error.

rspconfig is not supported for genpdu; PDU linking and BCM2/PMC power
meters are out of scope.

Tested on Raritan PX4-5851-E7V2 (fw 4.2.10.5-50400, switched),
PX3-1901U-N1 and PX3-1901U-N1A6 (fw 4.0.20.5-49038, metered), and
PX2-1901U-N1A6 (fw 4.0.20.5-49038, metered).

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