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.
Do we want to give descriptive error messages here if proxying is rejected either because it's not allowed at all, or because it's not allowed from the host trying to proxy?