Allow for commands that use positional parameters to add/remove attributes
Commands that modify a single multivalued attribute of an entry should use
positional parameters to specify both the primary key and the values to
add/remove. Named options are redundant in this case.
The `--certificate option` of `*-add/remove-cert` commands was turned
mandatory to avoid EmptyModlist when it is omitted.
Reviewed-By: David Kupka <email@example.com>
Reviewed-By: Jan Cholasta <firstname.lastname@example.org>