command_handler_t: make cmd parameter const

Prevents the command name from being modified in command handlers.
Again, this has cascading effects, but the patches are fairly minimal.

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, unsigned argc, ##extra)
+				const char *cmd, char **args, unsigned argc, ##extra)
 
 /**
  * Use this to macro to call a command helper (or a nested handler).