help_print_optstring: Better output on some old systems.

Some very old printf implementations return 0 on success instead of the
number of bytes written.  We should never see a return of 0 normally,
so we can improve the output to be less of a garbled mess by printing a
newline if that happens (same as the error case).  This is a simple
tweak that should have virtually no impact on modern systems.

The tests are adapted to verify the new behaviour, although we'll mark
the result as "skipped" (assuming it is correct) instead of "passed"
since it's only sortof working.