Support Apache 2.4 client IP address lookups

The Apache 2.4 API replaces the conn_rec->remote_ip member with
conn_rec->client_ip or conn_rec->useragent_ip (per
http://httpd.apache.org/docs/2.4/developer/new_api_2_4.html). client_ip
seems to be the best choice for us, to avoid everything being logged
from the load balancer or proxy instead of the client's address.

This code detects the version of Apache and uses client_ip if it is 2.4
or greater.

Signed-off-by: Dan Scott <dscott@laurentian.ca>
Signed-off-by: Bill Erickson <berick@esilibrary.com>