Improve diagnostics of lls_check_arg_count().

The old code could return an error message like

	at least 1 non-option args required, 0 given

even if exactly 1 arg is required. So check this case and print

	exactly 1 non-option args required, 0 given

instead. The check for the upper bound has the same issue. It is
considered a bug in the application if min_argc > max_argc, so check
for this case as well.