shutdown_halt: make sure that watchdog timer is stopped

The point of halt is to keep the machine in limbo.

Reviewed by:	kib
MFC after:	2 weeks
Differential Revision: https://reviews.freebsd.org/D21222

sys/kern/kern_shutdown.c

@@ -592,6 +592,9 @@ shutdown_halt(void *junk, int howto)
 		printf("\n");
 		printf("The operating system has halted.\n");
 		printf("Please press any key to reboot.\n\n");
+
+		wdog_kern_pat(WD_TO_NEVER);
+
 		switch (cngetc()) {
 		case -1:		/* No console, just die */
 			cpu_halt();