* Correctly signal broken backend connections up the chain also for the ajp

* Correctly signal broken backend connections up the chain also for the ajp
  backend (see also r357461). Furthermore move common code in mod_proxy_http.c
  and mod_proxy_ajp.c into a new function (ap_proxy_backend_broke) in
  proxy_util.c.

  modules/proxy/mod_proxy_ajp.c : Signal broken backend connection for ajp backend
  modules/proxy/proxy_util.c    : Add ap_proxy_backend_broke
  modules/proxy/mod_proxy_http.c: - Use ap_proxy_backend_broke
                                  - Return DONE also if backend broke
  modules/proxy/mod_proxy.h     : Add declaration of ap_proxy_backend_broke

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@365374 13f79535-47bb-0310-9956-ffa450edef68