Merge: curl: fix concurrent requests and intro shortcut services for scripts

This PR brings 2 main changes:

* The handle to a Curl request is now kept by the `CurlRequest` instances, instead of being shared by all requests. This makes it possible to execute concurrent HTTP requests.

* Intro two new services on `Text` to execute simple HTTP requests. While these new services are useful to simple scripts, if more control is needed you should still use `CurlHTTPRequest`.

  `Text::http_get` makes a GET request and returns the response body.

  `Text::http_download` makes a GET request and saves the response body to a file.

Pull-Request: #2731
Reviewed-by: Jean Privat <jean@pryen.org>