HDFS-6838. Merge r1617361 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1617363 13f79535-47bb-0310-9956-ffa450edef68
HDFS-6582. Merging change r1617366 from trunk

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1617369 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-10835. Implement HTTP proxyuser support in HTTP authentication client/server libraries. (tucu)

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