[Bro-Dev] changing Notice::policy mechanism

[Siwek, Jonathan Luke]

>  2. Adding a "break"-like keyword to halt policy execution (only valid in policy handlers).
> 
>  3. No return value(s) when calling policy handlers.

Just an alternative idea I had -- maybe they could be required to return a boolean value that's used internally to determine whether to keep traversing the vector of handler bodies.  Then you don't need to add a new keyword for the halt/break functionality.

    Jon