This factors out the HTTP proxy functionality from net/http,
with a view to vendoring it into net/http later.

See discussion in https://go-review.googlesource.com/c/go/+/68091

Change-Id: I8df8a92a13bca03504edd24b71a9a184f290b87d
Reviewed-on: https://go-review.googlesource.com/76910Reviewed-by: roger peppe <rogpeppe@gmail.com>