substitute-binary: Pass `filtered-port' an unbuffered port.

This fixes a bug whereby `read-response' would read more than just the
response, with the extra data going into the port's buffer; the
"bzip2 -dc" process spawned by `filtered-port' would not see the those
buffered data, which are definitely lost, and would bail out with
"bzip2: (stdin) is not a bzip2 file."

* guix/utils.scm (filtered-port): Document that INPUT must be
  unbuffered.
* guix/web.scm (http-fetch): Add `buffered?' parameter.  Call
  `open-socket-for-uri' explicitly, and call `setvbuf' when BUFFERED? is
  false.  Pass the port to `http-get'.  Close it upon 301/302.
* guix/scripts/substitute-binary.scm (fetch): Add `buffered?'
  parameter.  Pass it to `http-fetch'; honor it for `file' URIs.
  (guix-substitute-binary): Call `fetch' with #:buffered? #f for port RAW.
* tests/utils.scm ("filtered-port, file"): Open FILE as unbuffered.

guix/scripts/substitute-binary.scm

@@ -117,15 +117,17 @@ pairs."
           (else
            (error "unmatched line" line)))))
 
-(define (fetch uri)
+(define* (fetch uri #:key (buffered? #t))
   "Return a binary input port to URI and the number of bytes it's expected to
 provide."
   (case (uri-scheme uri)
     ((file)
      (let ((port (open-input-file (uri-path uri))))
+       (unless buffered?
+         (setvbuf port _IONBF))
        (values port (stat:size (stat port)))))
     ((http)
-     (http-fetch uri #:text? #f))))
+     (http-fetch uri #:text? #f #:buffered? buffered?))))
 
 (define-record-type <cache>
   (%make-cache url store-directory wants-mass-query?)
@@ -423,7 +425,7 @@ indefinitely."
        (format #t "~a~%" (narinfo-hash narinfo))
 
        (let*-values (((raw download-size)
-                      (fetch uri))
+                      (fetch uri #:buffered? #f))
                      ((input pids)
                       (decompressed-port (narinfo-compression narinfo)
                                          raw)))