Add RemoteIP module support for behind LB

If REMOTEIP_INTERNAL_PROXY is provided (usually a space separated list of proxy cidrs) then mod_remoteip is enabled. This results in the REMOTE_HOST being obtained from the X-Forwarded-For header, if available.

When processing the header (from right to left) any IPs in REMOTEIP_INTERNAL_PROXY are skipped.

Closes: #39 (closed)