with the following modifications to the initial patch:
- rename SHOLD and P_HOST to SSUSPEND and P_SUSPEND to avoid confusion with
PHOLD()
- don't deal with SSUSPEND/P_SUSPEND in fork1(), if we come here while
scheduler is suspended we're forking proc0, which can't have P_SUSPEND set.
sched_suspend() suspends the scheduling of users process, by removing all
processes from the run queues and changing their state from SRUN to
SSUSPEND. Also mark all user process but curproc P_SUSPEND.
When a process has to be put in SRUN and is marked P_SUSPEND, it's placed in
the SSUSPEND state instead.
sched_resume() places all SSUSPEND processes back in SRUN, clear the P_SUSPEND
flag.
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