Really stupid AI of immigrants

[Cherry Soup]

1. Immigrants doesn't check, that house already to be inhabited by another immigrant. Because of that, they almost every time go after each other and slows down immigration basically. (This need not simple algorithm by the way, because two immigrants can fit into one empty high-level house)
2. Sometimes they go into infinite loop, when they decided to go into house, which not near the road, they go near him, say "nah, I don't need it", and go back to road, and then decides to inhabit this house again.
3. Sometimes they just don't see the new house places and decides to leave city.

[dalerank]

fix #308. Improved emigrant ai nearest house.

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