libhpcalcs: portable (Windows / MacOS X / Linux) connectivity kit library

[debrouxl]

The tests performed by critor on his Prime show that libhpcalcs still mostly works with the latest (revision 6030) firmware, so the foundation of the protocol clearly hasn't changed

* the ready check, get infos, send key and send chat operations still work;
* the get screenshot operation yields "unhandled packet format", I'll have to find out why based on the debugging logs produced by libhpcalcs.

I had started making some changes for skipping the metadata at the beginning of .hpprgm files, as well as for making the "receive backup" operation more reliable in the face of all those packet losses occurring with the previous firmware version.