Fix "migration due to not enough food"

[Corbeau-1]

Is your feature request related to a problem? Please describe.
It is not 100% certain, but currently, everything points to this: spontaneous migration is not possible if the target city doesn't have a free tile that a new population would work on, even if there is abundant surplus food. This 1. makes no sense, 2. makes it more difficult to track and adjust which cities can take additional migrants.

Describe the solution you'd like
Either just removing this limit and come what may(*), OR replacing it with a limitation that the target city must have at least +2 surplus food per turn.

(*) it was not unheard of people moving to city and starving there. Also, a city may have enough of food supply to maintain new population until action is taken.

Describe alternatives you've considered
Quit Freeciv and live the rest of my life in a monastery.

Additional context
I don't have a life, apparently.