followup issue to ZBX-1999

1. server and frontend should completely agree on what are valid symbols in item key names and parameters;

2. simple checks should be transformed from "key,params" syntax to the common "key[params]" one. upgrade path should be provided

3. manual should be verified at http://www.zabbix.com/documentation/1.8/manual/config/items#allowed_characters and http://www.zabbix.com/documentation/1.8/manual/config/items#zabbix_agent