[Freeciv-Dev] Artillery and sea units (PR#1476)

[rms@xxxxxxx]

Canons and other such land units ought to be able to attack sea units.
In reality they could do so.  A coastal defense battery's guns were
larger than field guns, but not larger than siege guns.

The guns on a destroyer are the same kind as the "artillery" unit
represents, only typically bigger, so the destroyer (and likewise the
cruiser and the battleship) should not be available before the
artillery unit is developed.