Nationality initialization fixes

1. A handful of type fixes for initNationalities
2. Preserve slave trade data in NGP (fixes #2858 (closed) per Pregmodder's comment)

Note that existing games have already lost the customization flag, and will still lose their custom nationalities in NGP.

changed title from Nationality initalization fixes to Nationality initialization fixes

changed the description

mentioned in commit a1d3e196

merged