* guix/scripts/substitute-binary.scm (%narinfo-cache-directory,
%narinfo-ttl, %narinfo-negative-ttl): New variables.
(with-atomic-file-output, object->fields, read-narinfo,
write-narinfo, narinfo->string, string->narinfo, lookup-narinfo): New
procedures.
(fetch-narinfo): Adjust to use `read-narinfo'.
(guix-substitute-binary): Ensure the existence of
%NARINFO-CACHE-DIRECTORY. Use `lookup-narinfo' instead of
`fetch-narinfo'.
* 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.
* daemon.am (libexec_PROGRAMS, nix_setuid_helper_SOURCES,
nix_setuid_helper_CPPFLAGS, nix_setuid_helper_LDADD): New variables.
* test-env.in: Set and export `NIX_SETUID_HELPER'.
* README (Installing Guix as non-root): New section.
* test-env.in: New file.
* configure.ac: Add it to `AC_CONFIG_FILES' and `commands-exec'.
* config-daemon.ac: Set and substitute `GUIX_TEST_ROOT'.
* Makefile.am (SCM_LOG_COMPILER, SH_LOG_COMPILER): Use it in lieu of
`pre-inst-env'.
* daemon.am (test_root): Remove
(AM_TESTS_ENVIRONMENT): Remove `TEST_ROOT'.
(clean-local): Use $(GUIX_TEST_ROOT); make files writable before
removing them.guix_test_root
* tests/guix-daemon.sh: Remove `NIX_' variable settings; don't launch
`guix-daemon'.
