Use stronger random number generation for TCP_ISSINCR and tcp_iss.

Reviewed by:	peter, jlemon

sys/netinet/tcp_seq.h

@@ -91,7 +91,7 @@ extern tcp_cc	tcp_ccgen;		/* global connection count */
  * number in the range [0-0x3ffff] that is hard to predict.
  */
 #ifndef tcp_random18
-#define	tcp_random18()	((random() >> 14) & 0x3ffff)
+#define	tcp_random18()	(arc4random() & 0x3ffff)
 #endif
 #define	TCP_ISSINCR	(122*1024 + tcp_random18())
 

sys/netinet/tcp_subr.c

@@ -178,7 +178,7 @@ tcp_init()
 {
 	int hashsize;
 	
-	tcp_iss = random();	/* wrong, but better than a constant */
+	tcp_iss = arc4random();	/* wrong, but better than a constant */
 	tcp_ccgen = 1;
 	tcp_cleartaocache();
 

sys/netinet/tcp_timewait.c

@@ -178,7 +178,7 @@ tcp_init()
 {
 	int hashsize;
 	
-	tcp_iss = random();	/* wrong, but better than a constant */
+	tcp_iss = arc4random();	/* wrong, but better than a constant */
 	tcp_ccgen = 1;
 	tcp_cleartaocache();