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.

Makefile.am

@@ -73,6 +73,7 @@ MODULES =					\
   guix/scripts/hash.scm				\
   guix/scripts/pull.scm				\
   guix/scripts/substitute-binary.scm		\
+  guix/scripts/authenticate.scm			\
   guix/scripts/refresh.scm			\
   guix.scm					\
   $(GNU_SYSTEM_MODULES)
@@ -172,6 +173,8 @@ EXTRA_DIST =					\
   srfi/srfi-64.scm				\
   srfi/srfi-64.upstream.scm			\
   tests/test.drv				\
+  tests/signing-key.pub				\
+  tests/signing-key.sec				\
   build-aux/config.rpath			\
   bootstrap					\
   release.nix					\