refactor content_retriever_xmlrpc()

content_retriever_xmlrpc() is basically a long switch{} statement, where we laboriously check to see if we're using a key, and then pass the parameters we got.

This is itching for refactoring, as currently it's hard to maintain and hard to extend.

I suggest we use func_get_args() and just pass on whatever we received -- the only reason we switch for each method call is to pass the variable number of arguments to xmlrpc().

The alternative would be to move the building of the connection arguments further up, and always call like this:

  $args = content_retriever_build_xmlrpc_args()
  content_retriever_xmlrpc($args, *what we'd pass here anway*)

but we're still looking at a call_user_func_array() because we don't know how many arguments we might be dealing with. (*sigh* why isn't PHP more like perl?)