Allow iterating over bif functions with result type vector of any.

This changes the internal type that is used to signal that a vector
is unspecified from any to void.

I tried to verify that the behavior of Bro is still the same. After
a lot of playing around, I think everything still should worl as before.

However, it might be good for someone to take a look at this.

addresses BIT-1144

src/Val.h

@@ -656,6 +656,8 @@ protected:
 	DECLARE_SERIAL(PatternVal);
 };
 
+// ListVals are mainly used to index tables that have more than one 
+// element in their index.
 class ListVal : public Val {
 public:
 	ListVal(TypeTag t);