core: add r->flushed flag and set it when the response is sent.

core: add r->flushed flag and set it when the response is sent.

By setting EOR->r->flushed in the core output filter, allow one to determine at
log_transaction hook time whether the request has been fully flushed through
the network, or not (network issue, filter error, n-th pipelined resposne...).

Introduce the ap_bucket_eor_request() helper to get the request bound to an EOR
bucket, and uses it in ap_core_output_filter() to mark the EOR's request just
before destroying it, after all the previous buckets have been sent.

While at it, rename the request_rec* member of struct ap_bucket_eor from "data"
to "r", which makes the code clearer (not to be confused with b->data).

Finally, add CustomLog format %F, showing "F" or "-" depending on r->flushed,
for admins to figure out for each request.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1876017 13f79535-47bb-0310-9956-ffa450edef68