Pause event handling while opening and closing devices

Pause event handling while opening and closing devices

Ludovic Rousseau found that crashes often occur if you close a device
while another thread is doing event handling.

Fix this by adding an internal control pipe, which the close routines
use to interrupt the event handler and obtain the event handling lock,
ensuring that no other thread is handling events while the device is
closed. After the close completes, it signals all the event handlers
to start up again using the usual mechanism.

Also modified libusb_open() to do a similar thing, so that event
handlers are interrupted in order to realise that a new poll fd has
appeared.