U2F trickery

In the last post I promised to stop threat analyzing. So here
is some dev again which I already started developing back
in 2014 and where I finally found some time to finish.
Its a small U2F stack with the APDU framing code based
on Googles U2F reference code. After reviewing a lot of other
U2F code (sigh!), I found this reference code comprehensive enough to be usable for myself and for PAM code.
It also builds on Darwin, but I didnt have time to test it
there.