Determine the rulesdir for the given account.

        Return None if not specified.

def rulesdir(self, acc=None):
        """
        Determine the rulesdir for the given account.

        Return None if not specified.
        """
        rootdir = self.rootdir()
        rulesdir = self.get('rulesdir', acc=acc, default=[])
        return os.path.join(rootdir, rulesdir) \
            if rootdir and rulesdir else None