Add an operserv/oper/modify privilege.

As exposed in [bug #1612](https://bugs.anope.org/view.php?id=1612), operserv/oper cannot be configured to grant read-only access to the command. If an oper has the operserv/oper command access, he can add/remove operators with at most the same privileges as himself. Without the privilege, he cannot use list or info.

This patch adds a new privilege operserv/oper/modify which is required to use operserv/oper/add and operserv/oper/del. An oper with the command access operserv/oper can only use operserv/oper/list and operserv/oper/info.

With the example conf, the opertypes Services Administrator and Services Root will have the same access as before, but Services Operator can only LIST/INFO.