Threading2

* posix: don't try to use sched_setaffinity for setting thread affinity.
It requires the os-level thread id which we do not know without hackery.

* ThreadGroup: use weakrefs internally to avoid creating memory leaks.
* SHLock: fix cleanup logic when acquire() times out

* rework SHLock class to avoid deadlocks, be fair and re-entrant.
* ensure that the thread addinity is set properly on thread startup.

* add SHLock class for shared/exclusive (also known as read/write) locks.

* fix silly typo when loading libc (it's a miracle previous versions
worked at all!)

* better CPUSet <-> bitmask conversion
* posix: dynamically calculate size of _cpuset struct.