Genie:  Don't store single-use value as static var

The NewProcess() call that creates the root process is only called once,
so we don't need to cache the result of MacFeatures::Has_Threads().  (On
PPC it's a constant anyway, so this was a pessimisation.)
Genie:  Throw ThreadManager_required, not ENOSYS
Genie:  Complain if Thread Manager isn't available
Merge branch 'threadfail'