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)
succeeds at usertests.c pipe test
usertest.c tests pre-emption kill()
swtch idles on per-CPU stack, not on calling process's stack fix pipe bugs usertest.c tests pipes, fork, exit, close
