Use useragent_ip instead of client_ip on Apache 2.4

Although <tt>r->connection->client_ip</tt> is the 2.4 equivalent of <tt>r->connection->remote_ip</tt> in Apache 2.2, for mod_auth_pubtkt's use case it's more appropriate to check against <tt>r->useragent_ip</tt>, which is the IP of the actual user if there is a transparent load balancer or proxy between the user and the server.

For more details: https://httpd.apache.org/docs/2.4/developer/new_api_2_4.html