Introduce an ap_get_useragent_host() accessor to replace the old

Introduce an ap_get_useragent_host() accessor to replace the old
ap_get_remote_host() in most applications, but preserve the original
behavior for all ap_get_remote_host() consumers (mostly, because we
don't have the request_rec in the first place, and also to avoid any
unintended consequences).

This accessor continues to store the remote_host of connection based
uesr agents within the conn_rec for optimization.  Only where some
other module modifies the useragent_addr will we perform a per-request
query of the remote_host.

(Committed previously, but backed out due to unrelated core.c changes)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1729901 13f79535-47bb-0310-9956-ffa450edef68