core: Introduce atomic type and operations

core: Introduce atomic type and operations

An atomic variable is useful for reference counting and is much less
overhead than accessing such a variable while holding a lock. To that
end, replace the libusb_device 'refcnt' variable with an atomic and use
the atomic operations to manipulate it. This removes the need for the
mutex in the libusb_device.

Also convert the 'attached' variable to an atomic as well. This variable
was previously accessed both while holding the libusb_device mutex and
not.

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