Implement syscall_exec via process_execute

src/userprog/syscall.c

@@ -110,10 +110,14 @@ syscall_exit (int status)
 }
 
 static pid_t
-syscall_exec (const char *cmd_line UNUSED)
+syscall_exec (const char *cmd_line)
 {
-  //TODO
+  /* To check the end we would need to traverse the null-terminate string,
-  return 0;
+     which is equally unsafe as just leaving process_execute to do it. */
+  cmd_line = validate_user_pointer (cmd_line, 1);
+  if (cmd_line == NULL)
+    thread_exit ();
+  process_execute (cmd_line);
 }
 
 static int