Rewrite permissions to use map meta cap

It's a better approach than filtering user_has_cap. See discussion in http://core.trac.wordpress.org/ticket/22415

Start by reading the discussion in WordPress Trac ticket 22415, since the issue provides no file or test entry point. Determine the permission behavior that must move from filtering user_has_cap to using map_meta_cap; done means the permissions follow that approach and existing behavior remains covered by tests.

Written by the indexing model from the issue text.