mod_proxy_http: Fix 100-continue deadlock for spooled request bodies. PR 63855.

mod_proxy_http: Fix 100-continue deadlock for spooled request bodies. PR 63855.

Send "100 Continue", if needed, before fetching/blocking on the request body in
spool_reqbody_cl(), otherwise mod_proxy and the client can wait for each other,
leading to a request timeout (408).

While at it, make so that ap_send_interim_response() uses the default status
line if none is set in r->status_line.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1868576 13f79535-47bb-0310-9956-ffa450edef68