Add option not to send&clear response headers in ap_send_interim_response.

We'll need this option to fix PR#43711, and ap_send_interim_response
is fortunately too new an API to have made it into anything stable.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@588806 13f79535-47bb-0310-9956-ffa450edef68