1.0.25.39: thread start/stop fixes

1.0.25.39: thread start/stop fixes

- disable interrupts during create_thread

  ... to protect against signals (async unwinds, reentrancy, ...)
  during malloc, pthread code and to make the pinning of
  INITIAL-FUNCTION effective. Also add checks to
  new_thread_trampoline.

- make-thread: assert gc enabled (to prevent deadlocks)

- block blockables while holding LOCK_CREATE_THREAD lock

- make dying threads safe from interrupts