New scheduler.

Removed cli and sti stack in favor of tracking
number of locks held on each CPU and explicit
conditionals in spinlock.c.

Undo change from this morning that wasn't intended to get in.

Change fetchint, fetcharg, and putint to return -1 on error, 0 on success.
They had been returning 0 on error, 1 on success, but all the callers
were checking for return value < 0.

wakeup1() assumes you hold proc_table_lock
sleep(chan, lock) provides atomic sleep-and-release to wait for condition
ugly code in swtch/scheduler to implement new sleep
fix lots of bugs in pipes, wait, and exit
fix bugs if timer interrupt goes off in schedule()
console locks per line, not per byte

exit had acquire where I meant release
swtch now checks that you hold no locks

nesting cli/sti: release shouldn't always enable interrupts
separate setup of lapic from starting of other cpus, so cpu() works earlier
flag to disable locking in console output
make locks work even when curproc==0
(still crashes in clock interrupt)

fix race in schedule()

succeeds at usertests.c pipe test

usertest.c tests pre-emption
kill()

Linux 2.4 box using gcc 3.4.6 don't seem to follow the same
conventions as the i386-jos-elf-gcc compilers.
Can run make 'TOOLPREFIX=' or edit the Makefile.

curproc[cpu()] can now be NULL, indicating that no proc is running.
This seemed safer to me than having curproc[0] and curproc[1]
both pointing at proc[0] potentially.

The old implementation of swtch depended on the stack frame layout
used inside swtch being okay to return from on the other stack
(exactly the V6 you are not expected to understand this).
It also could be called in two contexts: at boot time, to schedule
the very first process, and later, on behalf of a process, to sleep
or schedule some other process.

I split this into two functions: scheduler and swtch.

The scheduler is now a separate never-returning function, invoked
by each cpu once set up.  The scheduler looks like:

	scheduler() {
		setjmp(cpu.context);

		pick proc to schedule
		blah blah blah

		longjmp(proc.context)
	}

The new swtch is intended to be called only when curproc[cpu()] is not NULL,
that is, only on behalf of a user proc.  It does:

	swtch() {
		if(setjmp(proc.context) == 0)
			longjmp(cpu.context)
	}

to save the current proc context and then jump over to the scheduler,
running on the cpu stack.

Similarly the system call stubs are now in assembly in usys.S to avoid
needing to know the details of stack frame layout used by the compiler.

Also various changes in the debugging prints.

user program that makes a blocking system call

disk interrupts (assuming bochs has a bug)

swtch idles on per-CPU stack, not on calling process's stack
fix pipe bugs
usertest.c tests pipes, fork, exit, close

pipes

initialize 2nd cpu's idt

curproc array

and plan 9 code, at least boots and gets into C code.