Revision 1877646

mod_proxy_http: handle Upgrade requests and upgraded protocol forwarding.

If the request Upgrade header matches the worker upgrade= parameter and
the backend switches the protocol, do the tunneling in mod_proxy_http.
This allows to keep the protocol to HTTP until the backend really
switches the protocol, and apply usual output filters.

When configured to forward Upgrade mechanism, we want the backend to be
able to announce its Upgrade protocol to the client (e.g. with 426
Upgrade Required response) and thus forward back the Upgrade header that
matches the one(s) configured in the worker upgrade= parameter.

modules/proxy/mod_proxy.h:
modules/proxy/proxy_util.c:
    ap_proxy_worker_can_upgrade(): added helper to determine whether a
    proxy worker is configured to forward an Upgrade protocol.

include/ap_mmn.h:
    Bump MMN minor for ap_proxy_worker_can_upgrade().

modules/proxy/mod_proxy.c:
    set_worker_param(): handle worker parameter upgrade=ANY as upgrade=*
    (should the "any" protocol scheme be something some day..).

modules/proxy/mod_proxy_wstunnel.c:
    proxy_wstunnel_handler(): use ap_proxy_worker_can_upgrade() to match
    the Upgrade header. Axe handling of upgrade=NONE, it makes no sense to
    Upgrade a connection if the client did not ask for it, nor to configure
    mod_proxy_wstunnel to use a worker with upgrade=NONE by the way.

modules/proxy/mod_proxy_http.c:
    proxy_http_req_t: add fields force10 (force HTTP/1.0) and upgrade (value
    of the Upgrade header sent by the client if it matches the configuration,
    NULL otherwise).
    proxy_http_handler(): use ap_proxy_worker_can_upgrade() to determine
    whether the request is electable for end to end protocol upgrading and set
    req->upgrade accordingly.
    terminate_headers(): handle Connection and Upgrade headers to send to the
    backend, according to req->force10 and req->upgrade set before.
    ap_proxy_http_prefetch(): use req->force10 and terminate_headers().
    send_continue_body(): added helper to send the body retained for end to
    end 100-continue handling.
    ap_proxy_http_process_response(): use ap_proxy_worker_can_upgrade() to
    match the response Upgrade header and forward it back if it matches the
    configured one(s). That is for 101 Switching Protocol obviously but also
    any other status code which is not overidden, at the backend wish. If the
    protocol is switching, create a proxy tunnel and run it, using the minimal
    timeout from the client or backend connection.

Github: closes #125