Feature request: support for `sched_setscheduler`

[eglimi]

There is currently no support for getting or setting the scheduling policies with sched_getscheduler and sched_setscheduler respectively. I use this on Linux with PREEMPT_RT to set realtime priorities and FIFO scheduling, currently with libc.

Other related parts are also not in rustix, e.g. to set the stack size (pthread).

Would you consider this in scope of rustix?