Small tweaks to the item db overhaul

- Consolidated format of the item-validation error messages.
- Changed some item validation errors to be non-fatal and default to
  safe values in case of error.
- Made item trade/item nouse override GM level an optional value
  (defaults to 100) - configuration fiel update pending.
- Added support for NULL in all the optional columns of the SQL item db
  - sql file update pending.

Signed-off-by: Haru <haru@dotalux.com>
Added ItemTradeRestriction enum

- To be used in a follow-up item_db.conf edit.

Signed-off-by: Haru <haru@dotalux.com>
Added ItemNouseRestriction enum

- To be used in a follow-up item_db.conf edit.

Signed-off-by: Haru <haru@dotalux.com>
Split Trade setting in the item DB into a group

- Item DB update pending

Signed-off-by: Haru <haru@dotalux.com>
Split Nouse setting in the item DB into a group

- Item DB update pending

Signed-off-by: Haru <haru@dotalux.com>
Added note to the generated SQL item DB files

- Tbhe note discourages manual edit of those files, since they're
  auto-genrated and any changes are bound to get lost in case they're
  edited.

Signed-off-by: Haru <haru@dotalux.com>
Changes to the db2sql plugin to make it more maintainable

Signed-off-by: Haru <haru@dotalux.com>
Updated db2sql converter to match the recent item db changes

Signed-off-by: Haru <haru@dotalux.com>