testing and real-life applications:
1) If you returned from the thread function, you got a segv instead of
calling _exit() with your return code.
2) clean up some bogus stack management. There was also an underflow
on function return.
3) when making syscalls, the kernel is expecting to have to leave space
for the function's return address. We need to duplicate this. It was
an accident that the rfork syscall actually worked here. :-/
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