Introduce a common package x/net/http/httpguts which can be vendored by
net/http to share detail implementations of the HTTP specification with
x/net/http2.

Updates golang/go#23908

Change-Id: Id5a2d51e05135436cf406c4c4d1b13fca7f84a32
Reviewed-on: https://go-review.googlesource.com/104042Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>