Initialize the usb library only once
Initialize the usb library only once

Initializing the hidapi library more than once is tricky. The hid_init()
function can be called multiple times, but the the hid_exit() function
will free the resources unconditionally, regardless of how many times
the hid_init() function has been called. The consequence is a premature
termination of the library.

To avoid this problem, the calls are reference counted. Note that this
workaround can't protect against calls made outside the libdivecomputer
code!

The libusb library doesn't suffer from this problem, because each
initialization returns a new context pointer. But for consistency, we
now also use a single reference counted libusb context.

henrik242/libdivecomputer,henrik242/libdivecomputer