thread: simplify 'free' process

thread: simplify 'free' process

GThread is freed using some very slightly confusing logic: if the thread
was created 'joinable', then the structure is freed after the join()
call succeeds (since we know the thread has exited).  If the thread was
not created 'joinable' then the free is when the thread quits (since we
know 'join' will not be called later).

Move to a straight ref-counting system: 1 ref owned by the thread and 1
extra ref if the thread is joinable.  Both thread quit and joining will
decrease the refcount by 1.