Copyright for new files

We have added several new files, not all of which are intended to be upstreamed:

1. src/circl/* -- not intended for upstream
2. crypto/tls/ech*.go -- not intended for upstream (at least not before RFC)
3. crypto/tls/delegated_credentials*.go (#28) -- not intended for upstream (at least not before RFC)
4. crypto/tls/tls_cf.go -- not intended for upstream

The question is how to write copyright information for these files. Things intended for upstream should probably be "Go authors", while things not intended for upstream should be "Cloudflare".