0.9.4.1: thread allocation

0.9.4.1: thread allocation
  * *CURRENT-THREAD* is now properly unbound (don't do bind_variable
    without unbind), the workaround from 0.9.3.75 is removed
  * also *CURRENT-THREAD* is temporarily bound to nil in the parent
    thread to avoid the child inheriting the value from the parent
    that could unnecessarily keep the parent thread object around
    until the child exited
  * free threads' interrupt_data when necessary
  * made all_threads_lock a mutex instead of a spinlock to speed
    start_the_world up
  * minor cleanups