Consistently require guns for dropping depth charges

Consistently require guns for dropping depth charges

The fire command requires at least one gun for depth charging, but
missions and return fire don't.  Has always been that way, except
between 4.0.6 and commit a3ad623 (v4.3.12), when depth charging worked
exactly like gun fire, and guns were consistently required.

Require guns for all ways to drop depth charges.