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

[debrouxl]

Thanks for the reminder. I had already thought of that feature request but forgot to add an item to the TODO list

I think that there are two steps in fulfilling that feature request:
Step 1: base functionality:
1) adding support for a "send_keys" operation, hpcalcs_calc_send_keys() and backends;
2) in test_hpcalcs, using strtok (once for initialization, then in a loop until NULL is returned), then for each token, strtoul with base 0 on each token (breaking upon error and upon invalid key code) to incrementally build an array of key codes, before passing that array to hpcalcs_calc_send_keys();

Level 2: refinements taking more time and left for future work:
1) adding a header file containing 4 * 51 definitions for the key codes (will take a bit of effort);
2) add an array of structs containing at least the key ID and the key name, stringified from the definition.
3) add a sorting functions by string name, so that binary search can be used;
4) add a key searching function, which parses the input string using strtoul with base 0, then if that fails, searches by string name into the sorted array, and returns a key ID or error;
5) use the key searching function in test_hpcalcs.