Clean up in-flight transfers and device handle when closing a device

Clean up in-flight transfers and device handle when closing a device

Any in-flight transfers should properly invalidate their references
to device handles that are being closed.  Additionally, they should be
removed from the transfer-in-flight list.  This is done with the events
lock held to protect against another thread processing the same transfer.
The events lock is initialized as a recursive mutex, because the device
close code might itself be called while an event is being handled.
Fixes #82.

[stuge: Trivial rework to reduce indenting]