Enhance handling of command line errors, and "--help" & "--version"

Enhance handling of command line errors, and "--help" & "--version"

Return with exit code 0 ("no error") when "--help" or "--version" was
used (this resulted in exit code 1, "error" before).

And exit with code 2 ("command line error") for all invalid command
line options, and show the error message on stderr (message was printed
to stdout before, and exit code was 1, "generic error").

This new behaviour is more in line with the GNU "coding standards",
see <https://www.gnu.org/prep/standards/html_node/_002d_002dhelp.html>.