MAPREDUCE-4374. Fix child task environment variable config and add support for Windows. Contributed by Chuan Liu.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1502046 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-HADOOP-7435-trunk-gd