Add reason for the last transfer to battery

[Jonher937]

# HELP apcupsd_last_transfer_on_battery_reason Reason for last transfer to battery.
# TYPE apcupsd_last_transfer_on_battery_reason gauge
apcupsd_last_transfer_on_battery_reason{reason="Input frequency out of range",ups="ups"} 1

$ apcaccess status | grep LASTXFER
LASTXFER : Input frequency out of range