Allow setting server variables separately from HTTP headers

[ks3]

I have an setup with Apache running as a reverse proxy, sending various user information as HTTP headers. To make things interesting, some sites require different headers than others, so the Apache RequestHeader directive is used to map only the required information to the needed header name.

To allow this to work better, and not pollute the back-end request with all session data in headers that will never be used, the the subprocess_env and headers_in handling need separated.