command_handler_t: make argc unsigned

The number of command arguments will always be 0 or more, so use
the right type in handlers.  This has a cascading effect up through
the layers, but the new COMMAND_HANDLER macros prevented total chaos.

src/helper/command.h

@@ -88,7 +88,7 @@ typedef struct command_context_s
  */
 #define __COMMAND_HANDLER(name, extra...) \
 		int name(struct command_context_s *cmd_ctx, \
-				char *cmd, char **args, int argc, ##extra)
+				char *cmd, char **args, unsigned argc, ##extra)
 
 /**
  * Use this to macro to call a command helper (or a nested handler).