edit.php private post filter does not restrict by user

[ryan]

edit.php?post_status=private can leak private post titles. We need to check if the user can read_private_posts. If not, other people's private posts should not be showned. The same restriction needs to be applied when counting the number of private posts.

[ryan]

Proposed: Add 'perm' private query argument that can be 'readable' or editable. Change WP_Query::get_posts() to check perm when querying by post_status. Change wp_counts_posts() to accept a 'perm' argument and do the same perm check when querying the number of posts.

[ryan]

(In [7109]) Add option to check caps when querying a particular post status. fixes #6052

[ryan]

(In [7112]) Add option to check caps when querying a particular page status. fixes #6052