Accept MIB labels for genpdu enumerated values

With PDU2-MIB loaded, SNMP.pm returns an enumerated INTEGER as its label
rather than its number: a PX4 answers inletSensorUnits with "amp" rather
than "2". That failed the numeric test, fell back to none(-1) and
dropped the unit suffix from every rvitals reading.

Add pdu2_enum, which accepts the number, the label, or the label(value)
form Net-SNMP produces with quick printing off, and use it for the
sensor units and for outletSwitchingState.

Also stop requiring the switching probe to read on or off. An outlet can
be in any SensorStateEnumeration state, and pdu2_get has already ruled
out an absent instance, so any answer proves the outlet is switchable.

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