Check X-Forwarded-For header (or another header, depending on configuration)

[jvdmr]

Currently mod_evasive only checks the requests's useragent_ip. This is useless when running behind a reverse proxy (or NAT firewall?).

Check if the X-Forwarded-For header exists and contains a valid ip address, then use that, falling back to the useragent_ip if the header is not usable.

Also allow to override which header to use through configuration.

(as mentioned in #54 and #46)