Add a new LogFormat directive, %c, that will log connection

Add a new LogFormat directive, %c, that will log connection
status at the end of the response as follows:
 'X' - connection aborted before the response completed.
 '+' - connection may be kept-alive by the server.
 '-' - connection will be closed by the server.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86814 13f79535-47bb-0310-9956-ffa450edef68