A......... [ZBX-8660] Improve item.create and item.update API.
- Refactor validation parts of method CItem::checkInput() into ::validateCreate() and ::validateUpdate(). Move other code to ::create() and ::update(), respectively. Remove method CItem::checkInput(). 
- Fix CItem::create() to use ::validateCreate().
- Fix CItem::update() to use ::validateUpdate().
- Add validation for updating items created from prototypes - check that only allowed fields are updated.