Change fairland island size probability distribution

Island size is randomly chosen from the interval [1..2*is+1], with
expected value is.  Use two dice to roll the size instead of one.
This makes extreme sizes much less likely.
(cherry picked from commit 29d2f5ccfe)

src/util/fairland.c

@@ -809,7 +809,7 @@ grow_islands(void)
 	secs = 0;
 	if (!place_island(c, &x, &y))
 	    return;
-	isiz = 1 + rnd(2 * is - 1);
+	isiz = 1 + rnd(is) + rnd(is);
 	do {
 	    ++secs;
 	    find_coast(c);