Made *all* enemies face forward in Ellesh mission.

I don't really understand why this code was implemented only for the
boss and not for anyone else. My only guess is it was designed to make
the level harder by letting enemies fire backwards while you can't,
but mine-droppers can already effectively do that.

The primary reason I decided to make sure they all face forward is I
was finding it to be nearly impossible to get through this level in
Nightmare difficulty; those ships being able to shoot backwards is
a HUGE advantage, because it's almost impossible to position yourself
in an area that's safe.

It may be that the unfair previous behavior was put in to compensate
for the AI's frankly asinine movement pattern. The random movement is
fine most places, but here, it's blatantly obvious that it's random,
and the enemy ships pay a steep price for it. What the ships should
be doing is positioning themselves so that they end up shooting or
dropping mines at the player. But the boss itself also has such a
positioning problem anyway; that should be properly fixed in the
future, so I might as well do the same for the smaller ships when
that time comes.

src/game.cpp

@@ -849,7 +849,8 @@ static void game_doAliens()
 				}
 
 				if ((game.area == MISN_ELLESH) &&
-						(aliens[i].classDef == CD_BOSS))
+						((aliens[i].classDef == CD_BOSS) ||
+						    game.difficulty != DIFFICULTY_ORIGINAL))
 					aliens[i].face = 0;
 
 				if ((aliens[i].flags & FL_DEPLOYDRONES) && ((rand() % 300) == 0))