HADOOP-9450. HADOOP_USER_CLASSPATH_FIRST is not honored; CLASSPATH is PREpended instead of APpended. Contributed by Chris Nauroth and Harsh J. (harsh)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1469215 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd
Merge remote branch 'origin/branch-2-gd' into fb-YARN-474-branch-2-gd