[fix] correct determination of the IP for the request

For correct determination of the IP to the request the function
botdetection.get_real_ip() is implemented.  This fonction is used in the
ip_limit and link_token method of the botdetection and it is used in the
self_info plugin.

A documentation about the X-Forwarded-For header has been added.

[1] https://github.com/searxng/searxng/pull/2357#issuecomment-1566211059



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>