daemon: Add `--no-substitutes'.

Suggested by Mark H. Weaver.

* nix/nix-daemon/guix-daemon.cc (GUIX_OPT_NO_SUBSTITUTES): New macro.
  (options): Add `--no-substitutes'.
  (parse_opt): Add `GUIX_OPT_NO_SUBSTITUTES' case.
  (main): Leave `settings.substituters' empty when
  `settings.useSubstitutes' is false.

doc/guix.texi

@@ -295,6 +295,10 @@ The following command-line options are supported:
 Take users from @var{group} to run build processes (@pxref{Setting Up
 the Daemon, build users}).
 
+@item --no-substitutes
+Do not use substitutes for build products.  That is, always build things
+locally instead of allowing downloads of pre-built binaries.
+
 @item --cache-failures
 Cache build failures.  By default, only successful builds are cached.
 

nix/nix-daemon/guix-daemon.cc

@@ -65,6 +65,7 @@ builds derivations on behalf of its clients.";
 #define GUIX_OPT_DEBUG 9
 #define GUIX_OPT_CHROOT_DIR 10
 #define GUIX_OPT_LISTEN 11
+#define GUIX_OPT_NO_SUBSTITUTES 12
 
 static const struct argp_option options[] =
   {
@@ -90,6 +91,8 @@ static const struct argp_option options[] =
     },
     { "build-users-group", GUIX_OPT_BUILD_USERS_GROUP, "GROUP", 0,
       "Perform builds as a user of GROUP" },
+    { "no-substitutes", GUIX_OPT_NO_SUBSTITUTES, 0, 0,
+      "Do not use substitutes" },
     { "cache-failures", GUIX_OPT_CACHE_FAILURES, 0, 0,
       "Cache build failures" },
     { "lose-logs", GUIX_OPT_LOSE_LOGS, 0, 0,
@@ -152,6 +155,9 @@ parse_opt (int key, char *arg, struct argp_state *state)
 	  exit (EXIT_FAILURE);
 	}
       break;
+    case GUIX_OPT_NO_SUBSTITUTES:
+      settings.useSubstitutes = false;
+      break;
     case GUIX_OPT_DEBUG:
       verbosity = lvlDebug;
       break;
@@ -202,15 +208,20 @@ main (int argc, char *argv[])
 
       /* Use our substituter by default.  */
       settings.substituters.clear ();
+      settings.useSubstitutes = true;
+
+      argp_parse (&argp, argc, argv, 0, 0, 0);
+
+      if (settings.useSubstitutes)
+	{
 	  string subs = getEnv ("NIX_SUBSTITUTERS", "default");
+
 	  if (subs == "default")
 	    settings.substituters.push_back (settings.nixLibexecDir
 					     + "/guix/substitute-binary");
 	  else
 	    settings.substituters = tokenizeString<Strings> (subs, ":");
-
-
-      argp_parse (&argp, argc, argv, 0, 0, 0);
+	}
 
       if (geteuid () == 0 && settings.buildUsersGroup.empty ())
 	fprintf (stderr, "warning: daemon is running as root, so "