move dfs administrative interfaces to a separate command

The dfs adminsitrative commands (e.g. safemode, report, etc) should be separate from the dfs user commands. The proposal is to remove the following two commands:

bin/hadoop dfs -report
bin/hadoop dfs -safemode

The above two features will be available through a new command:

bin/hadoop dfsadmin -report
bin/hadoop dfsadmin -safemode

This is done so that there is a clean distinction between cluster administrative commands and users command.