WL#2944: Add proper syntax for DUMP commands

Affects: Server-7.1   —   Status: Un-Assigned   —   Priority: Medium

The "secret" DUMP commands should get a proper syntax (the useful ones at least)
and the result should be shown in the management client as well as the cluster log. 

something like 

ndb_mgm> i|ALL SHOW MEMORY USAGE
...

instead of 'ALL DUMP 1000'