osdn #43075: targeted to freeciv-3.1: Rename "Firepower1" combat bonus as "LowFirepower" to prepare to that the target firepower might be something else than exactly '1'.
This is to prepare later changes that the exact firepower value will be ruleset defined.
Ruleup will do the right thing in renaming the occurrences of the bonus when updating from 3.0 to 3.1.