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.
fe0cff14f6