[Nut-upsuser] Synthesize low batt (LB) fron SNMP UPS which does not support this?

[Greg Troxel]

I should add that my thinking is in part due to having a python script
that does the equivalent of upsc and reports status to Home Assistant
over MQTT.  If I had a UPS that didn't do LB, I would still want to see
LB reported via this mechanism at the right time.  With this support in
upsmon, that wouldn't happen.   I see a lot of the point of nut as
regularizing all UPS units into defined semantics so that one can manage
them without knowing about the unit specifically.