Shell: Use _exit() in the forked child if execvp() fails

If we can't find an executable to exec() after forking, we don't want
to run the atexit() handlers in the child process. Just use _exit()
instead to avoid this.

This was causing us to write out the shell history to ~/.history every
time a "command not found" error was printed.

Shell/main.cpp

@@ -754,7 +754,7 @@ static int run_command(const String& cmd)
  fprintf(stderr, "%s: Command not found.\n", argv[0]);
  else
  fprintf(stderr, "execvp(%s): %s\n", argv[0], strerror(errno));
- exit(1);
+ _exit(1);
  }
  ASSERT_NOT_REACHED();
  }