Allow setting an interval between losing line power and shutting down BEE.

The PCM has a mechanism for powering off ports based on input voltage, which is used to power down all but the components required to run the detector heater. This currently detects the clear switch from line power to battery power. It also detects dropping below a voltage which indicates that the batteries are getting exhausted. The current logic uses those to decide when to power off the BEE, etc.

We would like a variant on that. Define the switch from line to battery power as t0. After a configurable interval from t0, if line power has not been restored, power off the BEE, etc.

The BEE can independently mark t0, or at least a decent proxy for that. So it will know when its power will be cut off and thus be able to shut itself down properly or do any other cleanup tasks.