mod_logio: introduce new optional function ap_logio_get_last_bytes to get

total byte count of last request.

core: Use ap_logio_get_last_bytes to report more accurate byte counts in
mod_status if mod_logio is loaded. Without mod_logio, don't increment
counts for HEAD requests.

PR: 25656


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@823337 13f79535-47bb-0310-9956-ffa450edef68