mod_remoteip: Use r->useragent_addr as the root trusted address for verifying.
This fixes issue resulting in setting of bad useragent_ip when internal redirection has been generated as response to the request (typically as result of "ErrorDocument 40x").
In this case, the original request has been handled by mod_remoteip and its useragent_ip has been changed properly, but when internal redirection to ErrorDocument has been generated later, the mod_remoteip's handler has been executed again with *the same* c->client_addr as in the original request. If c->client_addr IP is trusted, this results in bad useragent_ip being set.
When using r->useragent_addr as the root trusted address instead of c->client_addr, the internal redirection uses the first non-trusted IP in this particular case, so it won't change the r->useragent_ip during the internal redirection to ErrorDocument.