- Linux: Races are suspected to occur in multi-threaded applications when
  cancelling transfers while there is an event handler in progress.
  Variables like reap_action need protection. This needs some careful thought
  and testing.

- Linux: data can be lost when transfers are cancelled. libusb incorrectly
  assumes that transfers are always empty when they are reaped after
  cancellation. It should move the data to the correct point in the buffer --
  while still inconvenient for the application developer, it's better than
  losing it altogether.