* guix/scripts/substitute-binary.scm: Rename to...
* guix/scripts/substitute.scm: ... this.  Adjust module name, entry
  point, comments, and help string accordingly.
* nix/scripts/substitute-binary.in: Rename to...
* nix/scripts/substitute.in: ... this.
* pre-inst-env.in (NIX_SUBSTITUTERS): Adjust accordingly.
* tests/substitute-binary.scm: Rename to...
* tests/substitute.scm: ... this.  Adjust references to (guix scripts
  substitute) accordingly.
* guix/ui.scm (show-guix-help)[internal?]: Change "substitute-binary" to
  "substitute".
* Makefile.am (MODULES, SCM_TESTS): Adjust to file renames.
* daemon.am (nodist_pkglibexec_SCRIPTS): Likewise.
* config-daemon.ac: Likewise.
* guix/tests.scm (call-with-derivation-narinfo): Adjust comments and
  docstring.

* guix/scripts/substitute-binary.scm: New file.
* Makefile.am (MODULES): Add it.
* nix/scripts/substitute-binary.in: New file.
* config-daemon.ac: Produce nix/scripts/substitute-binary.
* daemon.am (nodist_pkglibexec_SCRIPTS): Add
  nix/scripts/substitute-binary.
* guix/store.scm (substitutable-path-info): Use the
  `query-substitutable-path-infos' RPC.
* nix/nix-daemon/guix-daemon.cc (main): Honor `NIX_SUBSTITUTERS'.
* pre-inst-env.in: Set `NIX_SUBSTITUTERS'.
* test-env.in: Leave `NIX_SUBSTITUTERS' unchanged.  Set
  `GUIX_BINARY_SUBSTITUTE_URL, and create
  $NIX_STATE_DIR/substituter-data.
  Run `guix-daemon' within `./pre-inst-env'.
* tests/store.scm ("substitute query"): New test.