Added a lighter version of the cargo ship buff to other difficulties.

Assisted difficulty multiplies the shield of the cargo ships in
Urusor by 4 so that stray shots are less likely to destroy them.
I've added a lesser version of this buff for these ships specifically
(rather than all friendly ships) on other difficulties: shield is
multiplied by 3. This is just enough to survive a stray missile, a
little stray laser fire, or a little stray plasma fire without being
destroyed (but sustained or direct fire will still destroy the ship).

Of course, this is not applied to Classic difficulty. It is also
excluded from Hard and Nightmare difficulties.

src/alien.c

@@ -84,6 +84,15 @@ void alien_nerf(int index)
 			aliens[index].maxShield /= 2;
 		}
 	}
+	else if ((game.difficulty != DIFFICULTY_ORIGINAL)
+		&& (game.difficulty < DIFFICULTY_HARD)
+		&& (game.area == MISN_URUSOR)
+		&& (aliens[index].classDef == CD_CARGOSHIP))
+	{
+		// Lighter version of the Urusor cargo ship shield buff.
+		aliens[index].shield *= 3;
+		aliens[index].maxShield *= 3;
+	}
 }
 
 void alien_defs_init()