Fix admin/cli library interface.

When adding to the admin/cli library, it is necessary to update Usage(), and necessary to manually add the functionality to any derived command line tools (e.g. demo/admin_cli).

This issue is to clean this up.

Suggested resolution is to restructure the cli library to define a command struct or interface, so that each operation's usage line can be defined next to its implementation. Then it can export a collection of known commands for derived tools to use.