improve CGI environment handling

rewrote environment handling for REST requests (currently GET only)
now environment will be based on actual request data and headers
all sensitive headers will not be exposed to user application
more standards-compliant handling of SERVER_NAME and SERVER_PORT
Yep. Sys-meta and X-Backend are Swift ones (we do not want to expose them), x-auth is auth key we don't want to expose it in case where users run not their own executable, and content-length + content-type are just exposed differently.