Rename 'guix substitute-binary' to 'guix substitute'.

* 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.