* Move handling of backends that broke after the headers have been sent

* Move handling of backends that broke after the headers have been sent
  into the proxy handler of mod_proxy.

  This patch still sets r->connection->aborted to 1 which is currently
  vetoed by Roy. Moving it from the scheme handler to the proxy handler
  should ease the reimplementation of this, as the scheme handlers only
  needs to return PROXY_BACKEND_BROKEN to signal the above situation to
  the proxy handler.

  mod_proxy.h: Add define for PROXY_BACKEND_BROKEN
  mod_proxy.c: Handle PROXY_BACKEND_BROKEN in proxy handler
  mod_proxy_http.c: Sent back PROXY_BACKEND_BROKEN if backend broke
  after we sent the headers.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@355823 13f79535-47bb-0310-9956-ffa450edef68