Revision 1682544

core, modules: Avoid error response/document handling by the core if some
handler or input filter already did it while reading the request (causing
a double response body).

Submitted by: ylavic
Backports: r1482522 (partial, ap_map_http_request_error() things only!),
           r1529988, r1529991, r1643537, r1643543, r1657897, r1665625, 
           r1665721, r1674056
Reviewed by: ylavic, minfrin, wrowe