Add missing access modifiers to methods in `wp_xmlrpc_server`. Add a magic `__call()` method for BC.

See #27881, #22234.

Built from https://develop.svn.wordpress.org/trunk@28515


git-svn-id: http://core.svn.wordpress.org/trunk@28341 1a063a9b-81f0-0310-95a4-ce76da25c4cd