Improve error messages from kadmin change_password

Improve error messages from kadmin change_password

The checks for missing option arguments were dead code, because the
loop condition requires at least two remaining arguments.  Instead
check for at least one argument with a leading "-", and check for too
many or too few arguments after the loop.  Add an initial message for
unrecognized options.

[ghudson@mit.edu: adjusted logic to improve mesages in more cases]