Add configurable DEADTIME from upsmon.conf in Services->UPS

Problem/Justification

Although I think this is more versatile:

Out of respect of being told there would be maintenance burden associated with it, in my particular case I need to modify DEADTIME.

Some UPS units (CP1500PFCLCD) have very slow reporting periods, which can make them appear dead to NUT with its default value for this variable after some time has elapsed. Once NUT consideres a UPS dead it seems to ignore it from that point forward, regardless of if its status changes at a later time, aside from a full reset.

This leads to nuisance warnings in SCALE that prevent one from knowing is their UPS is actuallt down, and may interfere with graceful shutdown of the UPS.

Currently I have to do this through a pre-init script, which while not the end of the world definitely makes it feel more like a hack.

Impact
Allows users with affected UPS units to properly configure NUT to work with them.

Adds one more field to the UPS service menu, which shouldn’t be to cumbersome.

User Story
As above. Add value and have UPS function properly.