Factor out common torpedo fire code into shp_torp()
This takes care of a number of bugs / inconsistencies:
* Submarines with zero mobility could interdict. Change to require
positive mobility.
* Submarines with zero firing range could not interdict. Fix by
dropping the test from perform_mission(). No ships in the stock
game are affected.
* Submarines without capability torp could fire return torpedoes and
interdict. Stock sbc, nm and msb were affected by the return fire
bug. Closes bug#950936.
* Shell resupply bugs: quiet_bigdef(), fire_torp() and
perform_mission() resupplied before checking all other requirements
and could thus get more shells than actually needed.
torp() no longer resupplies shells. It's hardly worth the bother, and
fire doesn't do it either.