bestfcom: Use fc.idealbvolts for calculating percent charge

'I have a Best Ferrups 3.1KVA and I noticed that the percent battery voltage
reported from upsc never gets above 69.9%.  I believe there is a mistake in the
driver.  The battery percentage is calculated based on the High Battery Alarm
Setpoint (59.6V) instead of the Ideal Voltage which is about 54V (4 batteries x
13.6V).  I examined the source code and found that the variable fc.fullvolts is
the result of querying parameter 67 from the UPS.  This parameter is defined
per Ferrups Tip 503 as: "The battery voltage at which the UPS sound a High
Battery alarm"  A better value to use in determining the percent battery
voltage would be fc.idealbvolts which is calculated in the driver source...

I propose the following patch to fix the battery.charge (% battery full) reading.'

Ref: http://article.gmane.org/gmane.comp.monitoring.nut.user/7891