great proxy logging cleanup:

    
  * remove "proxy:", "FCGI", etc. prefixes and pid which are now
    included in the error log format

  * propagate frontend request's logconfig to backend request
    
  * use ap_log_rerror where possible
    
  * remove obsolete APLOG_NOERRNO


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1203859 13f79535-47bb-0310-9956-ffa450edef68