Add option for trusted proxy

Hi,

you recently updated the IP detection to REMOTE_ADDR only – for good reason.
However not all reverse-proxy setups allow clean configuration to populate the correct value. (e.g. https://ww.wp.xz.cn/support/topic/logging-wrong-ip-address/ or https://ww.wp.xz.cn/support/topic/handling-ip-addresses-with-cdn/)

Suggestion to overcome that problem:
Add an option for “trusted proxy”.
For example a checkbox to let the user decide if the site is behind a proxy or not and optionally an IP address or range of trusted remotes. If the remote address matches this range, parse the X-Forwarded-For header.

Regards,
Stefan