Implement halt system call w/ S.

src/userprog/syscall.c

@@ -1,4 +1,5 @@
 #include "userprog/syscall.h"
+#include "devices/shutdown.h"
 #include <stdio.h>
 #include <syscall-nr.h>
 #include "threads/interrupt.h"
@@ -11,6 +12,9 @@ static void syscall_handler (struct intr_frame *);
    in size. */
 typedef uintptr_t syscall_function (uintptr_t, uintptr_t, uintptr_t);
 
+/* System Call Functions */
+static void halt (void);
+
 void
 syscall_init (void) 
 {
@@ -23,3 +27,9 @@ syscall_handler (struct intr_frame *f UNUSED)
   printf ("system call!\n");
   thread_exit ();
 }
+
+static void
+halt (void)
+{
+  shutdown_power_off ();
+}