LocalStore: Keep track of ultimately trusted paths

These are content-addressed paths or outputs of locally performed
builds. They are trusted even if they don't have signatures, so "nix
verify-paths" won't complain about them.

src/libstore/schema.sql

@@ -4,7 +4,9 @@ create table if not exists ValidPaths (
     hash             text not null,
     registrationTime integer not null,
     deriver          text,
-    narSize          integer
+    narSize          integer,
+    ultimate         integer, -- null implies "false"
+    sigs             text -- space-separated
 );
 
 create table if not exists Refs (