Reduced loopback MTU to 16k to work around a miriad of problems with it

set near or above 32k (likely caused by 16bit signed word overflow). 16k
actually (surprizingly) has higher performance than other values I tested.

sys/net/if_loop.c

@@ -31,7 +31,7 @@
  * SUCH DAMAGE.
  *
  *	@(#)if_loop.c	8.1 (Berkeley) 6/10/93
- * $Id: if_loop.c,v 1.5 1994/08/02 07:46:17 davidg Exp $
+ * $Id: if_loop.c,v 1.6 1994/08/08 12:07:25 davidg Exp $
  */
 
 /*
@@ -76,7 +76,7 @@
 #ifdef TINY_LOMTU
 #define	LOMTU	(1024+512)
 #else
-#define LOMTU	65532
+#define LOMTU	16384
 #endif
 
 struct	ifnet loif;