HDFS-6039. Merging change r1573879 from trunk to branch-2.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1573881 13f79535-47bb-0310-9956-ffa450edef68
HDFS-6040. fix DFSClient issue without libhadoop.so and some other ShortCircuitShm cleanups (cmccabe)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1573885 13f79535-47bb-0310-9956-ffa450edef68
HDFS-6046. add dfs.client.mmap.enabled (cmccabe)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1573888 13f79535-47bb-0310-9956-ffa450edef68