mod_proxy_http: forward 100-continue.

mod_proxy_http: forward 100-continue.

Handle end-to-end 100-continue, according to RFC 7231, such that the client
request body is not read/forwarded (according to its "Expect:" header) until
the backend wants to receive it (with interim 100 continue response), or never
forwarded if the backend provides a (non-interim) response and doesn't need
the client body at all.

This is achieved by filling the header_brigade in ap_proxy_http_prefetch()
and letting ap_proxy_http_request() determine whether it should forward that
brigade only (with the "Expect: 100-continue" specified by the client or added
according to "ping=" configuration), or forward the whole body for the usual
case (as before).

When 100-continue expectation is in place, the body is actually forwarded by
ap_proxy_http_process_response() when/if a "100 continue" response is sent by
the backend, otherwise the body is discarded; a future enhancement could make
so that in a balancer configuration, the body could be forwarded to another
balancer member depending on the status/error from the backend.

So stream_reqbody_cl() and stream_reqbody_chunked() functions are adapted to be
called by either ap_proxy_http_request() or ap_proxy_http_process_response(),
while spool_reqbody_cl() still spools the body in ap_proxy_http_prefetch() thus
before the backend is connected/reused to avoid inactivity on the connection
for the prefetch time (the prefetched body is also forwarded according to the
100-continue expectation, though).

Also, since the brigades and other runtime objects now need to be shared by the
ap_proxy_http_*() functions chain, a proxy_http_req_t struct/context is created
from the start and passed to them as (the single) argument. This is also a good
candidate for a future async baton, if we wanted to let the MPM event wait for
connection data for us at any stage and be called back ;)

Finally, ap_send_interim_response() is modified to correcly handle 100 continue
responses once, and take care of clearing r->expecting_100 only for them.

PR 60330.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1836588 13f79535-47bb-0310-9956-ffa450edef68