cobcd: Better error message for KeyboardInterrupt, cobcd-st: ensure no breakage from mising xxd

Commit 1: During testing there were some issues that led to a long running compile, when interrupting it there was no clue what was actually done.

Commit 2: I've got a report about broken cobcd-st compilation; in the end it showed to be a missing xxd redirect its output to a generated header file, which is then empty so the build goes on but misses definitions. (side note: thanks for letting me learn about xxd that way, that's quite useful!)

No-commit, because of different base (weak it is, my git foo): The cobcd script needs a fix, version 4.28.2 is broken for make install PYTHON=... because feab8ef3 introduced a change to the first line of "cobcd" - this file must start with the "python3" line, otherwise the auto-adjustment in make install PYTHON=-... does not work.

I guess that's an issue that came from use of the "make2" script, which I think is not necessary any more as we changed the tests to call the script not as executable any more but with the python binary, passing it as option. If this is the case I suggest to remove the make2 script along with changing the interpreter line in those two scripts back.