The DFU specification specifies that only one of the DFU interfaces has to
export a functional descriptor; I assumed they all had to. Adding support
for this kind of device rapidly turned into a massive restructure and it was
all too complicated anyway.
Reorganise the code so that we can support these kinds of devices and clean up
the API so it's sane and easy to use. This also allows us to generate the
GObject introspection GIR and to also install libdfu as a shared library.
If you've got any comments about the API, please shout now as when 6.0 is
released it will become API and ABI stable.
