OS161 pid Management

There are many way to manage each process's pid. Here is the way I do it.

I decided to make minimal modification to $OS161_SRC/kern/thread/thread.c, in case anything is ruined. So I only add two things to the thread module. One is I add a t_pid field to struct thread so that getpid system call is trivial. Another is I add a call of pid_alloc in thread_alloc to initialize new thread's t_pid. That's it. No more touch on the thread module.

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