web: Backport chunked encoding support for Guile <= 2.0.5.

* guix/web.scm (when-guile<=2.0.5): New macro.
  (read-chunk-header, read-chunk, read-chunk-body,
  make-chunked-input-port, read-response-body*)[when-guile<=2.0.5]: New
  procedures.
  (http-fetch): Clarify message when (not data).