add command usage, separate from help

Adds the usage command, to display usage information for commands.
The output for this command will remain erronenously empty until
commands are updated to use these new coventions.

src/helper/command.h

@@ -160,6 +160,7 @@ struct command
 {
 	char *name;
 	const char *help;
+	const char *usage;
 	struct command *parent;
 	struct command *children;
 	command_handler_t handler;
@@ -199,6 +200,8 @@ struct command_registration {
 	command_handler_t handler;
 	enum command_mode mode;
 	const char *help;
+	/// a string listing the options and arguments, required or optional
+	const char *usage;
 };
 
 /// Use this as the last entry in an array of command_registration records.