MFC r338993:

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

git-svn-id: svn://svn.freebsd.org/base/stable/9@339191 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f