Allow an MTU of 65535 bytes to be set via TUN[SG]IFINFO. This requires

changing the type on the mtu field in struct tuninfo from short to
unsigned short.
This is used, for example, by packetdrill to test with MTUs up to the
maximum value.

Differential Revision:	6452

sys/net/if_tun.h

@@ -25,11 +25,11 @@
 #define	TUNMTU		1500
 
 /* Maximum receive packet size (hard limit) */
-#define	TUNMRU		16384
+#define	TUNMRU		65535
 
 struct tuninfo {
 	int	baudrate;		/* linespeed */
-	short	mtu;			/* maximum transmission unit */
+	unsigned short	mtu;		/* maximum transmission unit */
 	u_char	type;			/* ethernet, tokenring, etc. */
 	u_char	dummy;			/* place holder */
 };