uspace: rate monotonic scheduling doesn't actually work #692
Description
There are occasional failures of tests/threads.0 at least on buster with RT kernel. The failure mode, line 11: got 0, expected 11 or 1 indicates that rate monotonic scheduling was violated, with the slower thread interrupting the faster one. This is not supposed to happen, as (under RT) the faster threads have strictly higher priority than slower threads. But also, for the non-rt case, I don't think the thread mutex ever worked as intended, since each thread has its own mutex but the old commit messages state that any thread will exclude another thread from executing (the intent was a single static mutex shared among all threads)