game: Add function game_init_land_ownership() to reinitialize ownership of complete map
savegame: Reinitialize land ownership after save game load, instead of storing it in the file
savegame: Remove some state that is unecessarily stored

Restore idle serf flag on load.
Do not store map values when they do not differ from the default value.
building,inventory: Clean up field names of structs and add documentation
interface: Count serfs moving out in separate field