MFC r338993:
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
Sponsored by: Mellanox Technologies