Limit XML-RPC method wp.getAuthors to only return user_id, user_login and display_name & add capability check (edit_posts)

[josephscott]

The wp.getAuthors method just returns all of the data provided by get_users_of_blog(), we should limit it to just specific useful information. In this case information that is needed and helpful for setting the post author: user_id, user_login and display_name.

Also add a capability check, at a minimum should be able to edit posts. If you can't even do that then there really isn't any reason to expose the list of authors on a blog.

(In [6498]) Limit what getAuthors exposes. Props josephscott for the patch and xknown for the find. fixes #5534 for 2.4

(In [6499]) Limit what getAuthors exposes. Props josephscott for the patch and xknown for the find. fixes #5534 for 2.3