misleading cmp error message

[Ron Olsson]

Whether cmp should allow duplicate -s options (or duplicate -l
options) isn't clear to me, but if not the error message could be
improved.

% cat /etc/redhat-release
Red Hat Linux release 9 (Shrike)
% /usr/bin/cmp -s -s x x
/usr/bin/cmp: options -l and -s are incompatible
/usr/bin/cmp: Try `/usr/bin/cmp --help' for more information.

Thanks.