Consider preserving more UPS battery by overriding battery charge 'low' level

Currently I am using NUT's default configuration for sending the `fsd` signal to switch computers off, which bases that call off the time the UPS status switches to `OB LB` from `OB`.

On my UPS, it seems like that is hardcoded to "10%" of runtime left, judging by my first end-to-end power loss test.

Allowing for shutdown time, that means my UPS would only be able to power a load in a true emergency for maybe 5-10 minutes.

I figure with the commercial power at the location I'm in, if there's an outage for more than 10-20 minutes, it's likely going to be a while, so why not preserve more UPS battery power?

Using `ignorelb` inside `ups.conf`, I could configure something like:

```
[server-room-rack]
...
ignorelb
override.battery.charge.low = 30
```

This would tell NUT to send the `fsd` shutdown command at 30% instead of 10% of UPS capacity.