Add a chanserv/access/list privilege

Replace the existing chanserv/access/list command access,
only used in cs_xop, by a privilege with the same name.

It which allows read-only access to cs_access, cs_flags, cs_xop
and cs_akick, while the already-existing chanserv/access/modify
privilege allows to modify them.

(See [bug #1619](https://bugs.anope.org/view.php?id=1619))