When multiple threads are involved receiving completion events in LibUSB

When multiple threads are involved receiving completion events in LibUSB
make sure there is always a master polling thread, by setting the "ctx_handler"
field in the context. Else the reception of completion events can stop.
This happens if event threads are created and destroyed during runtime.

Found by:		Ludovic Rousseau <ludovic.rousseau+freebsd@gmail.com>
PR:			231742
MFC after:		1 week
Approved by:		re (kib)
Sponsored by:		Mellanox Technologies