HDFS-5616. NameNode: implement default ACL handling. Contributed by Chris Nauroth.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-4685@1565845 13f79535-47bb-0310-9956-ffa450edef68
merge HDFS-4685 to HDFS-5899