Fix a -Wuninitialized warning by setting the socket to -1 and bump WARNS to 6

MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division

tools/regression/sockets/shutdown/Makefile

@@ -4,6 +4,6 @@
 
 PROG=	shutdown
 MAN=
-WARNS?=	2
+WARNS?=	6
 
 .include <bsd.prog.mk>

tools/regression/sockets/shutdown/shutdown.c

@@ -45,6 +45,8 @@ main(void)
 	int listen_sock, connect_sock;
 	u_short port;
 
+	listen_sock = -1;
+
 	/* Shutdown(2) on an invalid file descriptor has to return EBADF. */
 	if ((shutdown(listen_sock, SHUT_RDWR) != -1) && (errno != EBADF))
 		errx(-1, "shutdown() for invalid file descriptor does not "