param: make json_tok_ handlers all return command_result, rename to param_

Handers of a specific form are both designed to be used as callbacks
for param(), and also dispose of the command if something goes wrong.

Make them return the 'struct command_result *' from command_failed(),
or NULL.  

Renaming them just makes sense: json_tok_XXX is used for non-command-freeing
parsers too.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/options.c

@@ -1056,7 +1056,7 @@ static void json_listconfigs(struct command *cmd,
 	const jsmntok_t *configtok;
 
 	if (!param(cmd, buffer, params,
-		   p_opt("config", json_tok_tok, &configtok),
+		   p_opt("config", param_tok, &configtok),
 		   NULL))
 		return;