HDFS-5038. Backport several branch-2 APIs (FileSystem#newInstance, DFSClient#getNamenode, and FileStatus#isDirectory) to branch-1. Contributed by Jing Zhao.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-1@1522055 13f79535-47bb-0310-9956-ffa450edef68