multi: remove dns cache creation code from *add_handle

As it is done unconditionally in multi_init() this code will never run!
multi_init: moved init code here from add_handle

The closure_handle is "owned" by the multi handle and it is
unconditional so the setting up of it should be in the Curl_multi_handle
function rather than curl_multi_add_handle.
CURLM_ADDED_ALREADY: new error code

Doing curl_multi_add_handle() on an easy handle that is already added to
a multi handle now returns this error code. It previously returned
CURLM_BAD_EASY_HANDLE for this condition.