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.
Add an operserv/oper/modify privilege.

The operserv/oper command privilege allows to add operators
with fewer or the same privileges as ourself.
This new privilege allows to have operators who can use operserv/oper
to see the list of operators and their privileges, but cannot add new
operators.