Shell script rewrite

The Hadoop shell scripts have been rewritten to fix many long-standing bugs and include some new features. 

Incompatible changes are documented in the release notes. You can find them at https://issues.apache.org/jira/browse/HADOOP-9902.

There are more details available in the documentation at https://hadoop.apache.org/docs/r3.0.0/hadoop-project-dist/hadoop-common/UnixShellGuide.html. The documentation present at https://hadoop.apache.org/docs/r3.0.0/hadoop-project-dist/hadoop-common/UnixShellAPI.html will appeal to power users, as it describes most of the new functionalities, particularly those related to extensibility.