core: Stop the HTTP_IN filter from attempting to write error buckets

to the output filters, which is bogus in the proxy case. Create a
clean mapping from APR codes to HTTP status codes, and use it where
needed.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1482522 13f79535-47bb-0310-9956-ffa450edef68