Review request for JDK-8139895: Introduce GuardingDynamicLinkerExporter

[Attila Szegedi]

Please review JDK-8139895 "Introduce GuardingDynamicLinkerExporter" at <http://cr.openjdk.java.net/~attila/8139895/webrev.jdk9> for <https://bugs.openjdk.java.net/browse/JDK-8139895>

Remarks:
- DynamicLinkerFactory now loads the linkers in a doPrivileged block so that the lack of “dynalink.exportLinkersAutomatically” permission in the caller doesn’t preclude it from loading trusted linkers. This is consistent with how e.g. ScriptEngineManager uses ServiceLoader.
- DynamicLinkerFactory now has a new method "List<ServiceConfigurationError> getAutoLoadingErrors()” so that the user can inspect if some automatically loaded linkers failed to load. This is especially significant as the GuardingDynamicLinkerExporter can have some failure modes: it can lack the dynalink.exportLinkersAutomatically permission, it can return null from get() or any element of the returned element might be null

Thanks,
  Attila.