hotplug: Improve internal structure and deregistration mechanism

hotplug: Improve internal structure and deregistration mechanism

This commit shrinks the size of the internal hotplug callback structure
by removing unused fields, using the correctly sized types for matching
fields, and adding a new flags field whose bits control how the callback
structure should behave.

The hotplug callback handle ID counter has also been moved to the
context structure instead of being a global variable shared amongst all
contexts. This lets each context independently manage handle IDs and use
the maximum range of possible IDs.

Finally, the hotplug callback deregistration mechanism has been improved
to signal to the event handler that an explicit deregistration needs to
be handled. This removes the need to send a dummy hotplug message, which
was using an invalid libusb_hotplug_event value anyway that was causing
some compilers to complain.

Closes #373

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>