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).

src/helper/ioutil.c

@@ -214,9 +214,9 @@ COMMAND_HANDLER(handle_append_command)
 	config_file = fopen(args[0], "a");
 	if (config_file != NULL)
 	{
-		int i;
 		fseek(config_file, 0, SEEK_END);
 
+		unsigned i;
 		for (i = 1; i < argc; i++)
 		{
 			if (fwrite(args[i], 1, strlen(args[i]), config_file) != strlen(args[i]))