Change fire to always fire guns when the target is beyond range

Change fire to always fire guns when the target is beyond range

multifire() drops depth charges if the target is a submarine, else it
fires guns.  It fails if the target is out of range.  But players
could still find out whether the target is a sub then, because depth
charge shell use differs from gun fire shell use.  This loophole
existed before 4.0.6, and was reopened by commit a3ad623b (v4.3.12).

Change multifire() to always use guns if the target is out of range.

While there, treat failure from shp_dchrg() and shp_fire() the same,
so that the player can't distinguish the two cases.  Failure there
should not happen.