Merge from trunk:

Merge from trunk:

  *) mod_http2: added support for bootstrapping WebSockets via HTTP/2, as
     described in RFC 8441. A new directive 'H2WebSockets on|off' has been
     added. The feature is by default not enabled.
     As also discussed in the manual, this feature should work for setups
     using "ProxyPass backend-url upgrade=websocket" without further changes.
     Special server modules for WebSockets will have to be adapted,
     most likely, as the handling if IO events is different with HTTP/2.
     HTTP/2 WebSockets are supported on platforms with native pipes. This
     excludes Windows.
     New directive `H2ProxyRequests on|off` to enable handling
     of HTTP/2 requests in a forward proxy configuration.
     General forward proxying is enabled via `ProxyRequests`. If the
     HTTP/2 protocol is also enabled for such a server/host, this new
     directive is needed in addition.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1911979 13f79535-47bb-0310-9956-ffa450edef68