fix #H165 - sokoban "flaw" (trunk only)

Back in <email deleted> complained
that randomly generated giants in Sokoban would drop boulders when killed,
interfering with the puzzle solution.  He suggested that giants either
be disallowed or be generated without inventory in that dungeon branch.
This doesn't go that far; it just rejects giants on the first pass during
random monster selection, making them much less likely to appear there
but still allowing them if giant is chosen two times in a row.  (Existing
boulders aren't an issue here; giants can't pick them up in Sokoban.)

src/makemon.c

@@ -938,6 +938,7 @@ register int	mmflags;
 		 */
 		int tryct = 0;	/* maybe there are no good choices */
 		struct monst fakemon;
+
 		do {
 			if(!(ptr = rndmonst())) {
 #ifdef DEBUG
@@ -946,7 +947,11 @@ register int	mmflags;
 			    return((struct monst *) 0);	/* no more monsters! */
 			}
 			fakemon.data = ptr;	/* set up for goodpos */
-		} while(!goodpos(x, y, &fakemon, gpflags) && tryct++ < 50);
+		} while (++tryct <= 50 &&
+		    /* in Sokoban, don't accept a giant on first try;
+		       after that, boulder carriers are fair game */
+		    ((tryct == 1 && throws_rocks(ptr) && In_sokoban(&u.uz)) ||
+			!goodpos(x, y, &fakemon, gpflags)));
 		mndx = monsndx(ptr);
 	}
 	(void) propagate(mndx, countbirth, FALSE);