Revamp setting of extra headers and environment variables.

- Get rid of passenger_set_cgi_param.
- Introduced passenger_set_header.
- Introduced passenger_env_var.
- Environment variables are now passed to the request handler as a
  single base64-encoded string. This much improves performance.
Fix some Nginx integration tests and fix passenger_show_version_in_header.

The option is now only valid in the http context.
Fix Nginx handling of chunked request bodies
Fix forwarding of the Content-Type header
Fix support for base-URI requests
Fix forwarding of chunked app responses
Fix RequestHandler HTTP forwarding
Strip 100 Continue header from response when proxying through Apache or Nginx
Add trace points to Server, HttpServer and RequestHandler
Fix cEscapeString() unit tests
Minor refactorings

Rename ServerKit classes so that their function is clearer.