an "exclusion string" for the given option

        @type longname: C{str}
        @param longname: The long option name (e.g. "verbose" instead of "v")

        @type buildShort: C{bool}
        @param buildShort: May be True to indicate we're building an excludes
            string for the short option that corresponds to the given long opt.

        @return: The generated C{str}
        FNr_