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.
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