mod_remoteip: clear incoming RemoteIPProxiesHeader by default

[erkia]

RemoteIPProxiesHeader can be used to collect addresses of intermediate trusted proxies. Unfortunately, it is impossible to distinguish, if this header was set by mod_remoteip (can be trusted) or was set in a direct request, which did not pass any trusted proxies.

This change makes sure that RemoteIPProxiesHeader, if configured, is only set by mod_remoteip and is cleared for any requests, that do not come from trusted proxy.

[AlexAT]

Did you try RequestHeader unset headername early to unset the header before processing takes place?