Prevent access to constructors of restricted classes.
Return to use of public lookup for non-restricted method handles.

With the recent changes in JDK 7u25, there's no longer a need for a
zero-privilege delegate and we can return to using the public lookup
whenever unreflecting cacheable members (accessible caller-insensitive
members of classes in non-restricted packages).
Use simplified lookup API.
Fix use of Thread.getContextClassLoader() in DynamicLinkerFactory.

- Delay its invocation until absolutely necessary (as it is a security operation).
- Do it from a doPrivileged block