core: Treat timeout reading request as 408 error, not 400.

Log 408 errors in access log as was done in Apache 1.3.x.

PR: 39785
Submitted by: Nobutaka Mantani, Stefan Fritsch
Reviewed and added to by: Dan Poirier


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@820760 13f79535-47bb-0310-9956-ffa450edef68