Have the $fields argument for XML-RPC's wp.getPostType, getPostTypes,...

Have the $fields argument for XML-RPC's wp.getPostType, getPostTypes, wp.getTaxonomy and wp.getTaxonomies take the names of the post type or taxonomy object keys that will be returned.
 * capabilities becomes cap
 * object_types becomes object_type

fixes #17626.



git-svn-id: https://core.svn.wordpress.org/trunk@20696 1a063a9b-81f0-0310-95a4-ce76da25c4cd