Clean up the XML-RPC 3.4 methods with regards to consistency and future compatibility.

 * Limit returned taxonomy and post type objects to a specific type-cast whitelist of fields.
 * Add $filter and $fields to wp.getTaxonomies to match the functionality of wp.getPostTypes.
 * Prevent the post type from being chnaged in _insert_post().
 * Don't check post_type_supports() in _insert_post().
 * Align handling of sticky posts with those of core.
 * Force tags and categories to go through terms and terms_names, rather than tags_input and post_category.
fixes #20566. props nacin, maxcutler.



git-svn-id: https://core.svn.wordpress.org/trunk@20632 1a063a9b-81f0-0310-95a4-ce76da25c4cd