Core: Add HID and kernel detach capability detection for all backends

* Also remove Linux special case from xusb sample.
* Note that LIBUSBX_API_VERSION is incremented as a result of
  libusb_has_capability() returning nonzero rather than 1 when
  a capability is supported.
* A LIBUSB_CAP_HAS_HOTPLUG is also added, though it is currently
  not implemented on any platform
* Closes #102
libusbx 1.0.15-rc1

* Also fix missing header in release tarball