JCR-3802 : User Management: API for System Users (implementation for Oak)

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1618122 13f79535-47bb-0310-9956-ffa450edef68
OAK-2008 : authorization setup for closed user groups (exclude system users as defined by JCR-3802)

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1618123 13f79535-47bb-0310-9956-ffa450edef68