substitute-binary: Implement `--substitute'.

This allows build outputs to be transparently downloaded from
http://hydra.gnu.org, for example.

* config-daemon.ac: Check for `gzip', `bzip2', and `xz'.
* guix/config.scm.in (%gzip, %bzip2, %xz): New variable.
* guix/scripts/substitute-binary.scm (fetch): Return SIZE as a second value.
  (<narinfo>): Change `url' to `uri'.
  (make-narinfo): Rename to...
  (narinfo-maker): ... this.  Handle relative URLs.
  (fetch-narinfo): Adjust accordingly.
  (filtered-port, decompressed-port): New procedures.
  (guix-substitute-binary): Implement the `--substitute' case.
* tests/store.scm ("substitute query"): Use (%store-prefix) instead
  of (getenv "NIX_STORE_DIR").
  ("substitute"): New test.

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