addtional monster movement tweak

Introduce some variation in monster movement by adding and substracting
small random amounts to the fixed increment from the main monster table.  The
amount added is potentially bigger (average is +2) than the amount substracted
(average is +1), so monsters will tend to be slightly faster.  Probably not
noticeable except for super-slow monsters not being so glacially slow, since
net +1 is a bigger relative increase for such critters' low movement rates.
In practice, the noticeable effect will be that ordinary speed monsters will
occasionally get an extra move even if/when player keeps the hero unburdened.

     Possible extension (although I'm not planning to do it):  sort the
monster list by pending movement points, so that faster monsters move before
slower ones.  The random variation would become noticeable because groups of
same-speed monsters would alter their movement order depending upon who got
a bigger increment or smaller decrement on that turn.

doc/fixes35.0

@@ -1049,6 +1049,7 @@ bones files now include extra data to identify dead hero and reason for death
 dipping multiple potions in another potion may only dip part of their stack
 make being inside a stinking cloud (when not immune or resistant) become a
 	major trouble which is fixable by prayer
+introduce some variation in monster movement rates
 
 
 Platform- and/or Interface-Specific New Features

src/mon.c

@@ -454,8 +454,16 @@ struct monst *mon;
 	    /* average movement is 1.50 times normal */
 	    mmove = ((rn2(2) ? 4 : 5) * mmove) / 3;
 	}
-    }
+    } else
 #endif
+    if (mmove) {
+	/* vary movement points allocated to slightly reduce predictability;
+	   random increment (avg +2) exceeds random decrement (avg +1) by
+	   a small amount; normal speed monsters will occasionally get an
+	   extra move and slow ones won't be quite as slow */
+	mmove += rn2(5) - rn2(3);	/* + 0..4 - 0..2, average net +1 */
+	if (mmove < 1) mmove = 1;
+    }
 
     return mmove;
 }