Core/Commands: Implement possibility to unbind specific maps/difficulties with .instance unbind, based on svetilo12's idea, closes issue 3792

--HG--
branch : trunk

sql/updates/9768_world_command.sql

@@ -0,0 +1,3 @@
+DELETE FROM `command` WHERE `name` LIKE 'instance unbind';
+INSERT INTO `command` (`name`,`security`,`help`) VALUES
+('instance unbind',3,'Syntax: .instance unbind <mapid|all> [difficulty]\r\n  Clear all/some of player\'s binds');