Fix scripts/cmd
Document some structures in ex_commads.h, some refactoring

Refactoring:

- Moved followups element to Address structure
- Removed CommandNode.parent element
- Removed CommandArgument.arg.cmd, it is now replaced with CommandNode.children 
  completely for all node types
- Added CommandArgument.arg.unumber to represent unsigned integer, replaced some 
  kArgFlags usages with it