daemon: Implement signed archive import/export.
* guix/scripts/authenticate.scm, nix/scripts/guix-authenticate.in,
tests/signing-key.pub, tests/signing-key.sec: New files.
* po/POTFILES.in: Add 'guix/scripts/authenticate.scm'.
* guix/store.scm (dump-port): New procedure.
(process-stderr): Add 'user-port' optional parameter. Handle
the %STDERR-WRITE and %STDERR-READ cases as expected.
(import-paths, export-path, export-paths): New procedures.
* tests/store.scm ("export/import several paths", "import corrupt
path"): New tests.
* Makefile.am (MODULES): Add 'guix/scripts/authenticate.scm'.
(EXTRA_DIST): Add 'tests/signing-key.{pub,sec}'.
* daemon.am (libstore_a_CPPFLAGS)[-DNIX_CONF_DIR]: Change 'NIX_CONF_DIR'
to .../guix. Change 'OPENSSL_PATH' to 'guix-authenticate'.
* config-daemon.ac: Instantiate 'nix/scripts/guix-authenticate'.
* nix/nix-daemon/guix-daemon.cc (main): Augment $PATH to include
'settings.nixLibexecDir'.
* test-env.in: Export 'NIX_CONF_DIR' and 'NIX_LIBEXEC_DIR'. Populate
$NIX_CONF_DIR.
526382ff92