apache2: Apache 2.4 support.

Apache 2.4 renames the conn_rec struct member name from "remote_ip" to
"client_ip". Detect 2.4 and use COSIGN_CLIENT_IP macro to use the correct
struct member name for 2.2 and 2.4.

(cherry picked from commit aaea103f93c9364fa07a424218ec18cb715a2a20)
Add missing #undef HAVE_APACHE_CONN_CLIENT_IP to config.h.in.

Fix compiling for Apache 2.4.x.

(cherry picked from commit 46befd6b2db27bdb5798b3468ac0c2c640b02c1b)