Use X-Forwared-For when behind reverse-proxy

Allow the use of [X-Forwarded-For](http://en.wikipedia.org/wiki/X-Forwarded-For) if the header exists to allow reverse-proxies and loadbalancers to be set up in front of a bcfg2-server.

So far this diff only applies to the Builtin core. I could investigate applying it to the others as well, if there's interest in this PR.