Add restricted evaluation mode

If ‘--option restrict-eval true’ is given, the evaluator will throw an
exception if an attempt is made to access any file outside of the Nix
search path. This is primarily intended for Hydra, where we don't want
people doing ‘builtins.readFile ~/.ssh/id_dsa’ or stuff like that.

src/libexpr/eval.hh

@@ -135,6 +135,10 @@ public:
        already exist there. */
     bool repair;
 
+    /* If set, don't allow access to files outside of the Nix search
+       path or to environment variables. */
+    bool restricted;
+
 private:
     SrcToStore srcToStore;
 
@@ -155,6 +159,8 @@ public:
 
     void addToSearchPath(const string & s, bool warn = false);
 
+    Path checkSourcePath(const Path & path);
+
     /* Parse a Nix expression from the specified file. */
     Expr * parseExprFromFile(const Path & path);
     Expr * parseExprFromFile(const Path & path, StaticEnv & staticEnv);