[Freeciv-Dev] Tech cost patch v13 (PR#1082)

[Juha Litola <slave@xxxxxxxxxxxx>]

This is version 13 of tech cost patch.

This patch allows changing style of tech cost calculation. It also
allows techs to leak from civilization to another. Description of
modes from game.ruleset

tech_cost_style:
0 - Civ (I|II) style. Every new tech add researchcost to cost of
    next tech.
1 - Cost of technology is (1+num_parents)*researchcost, where
    num_parents == number of requirement for tech, counted
    recursively.
2 - Cost are read from tech.ruleset. Missing costs are generated by
    style 1.

tech_leakage:
0 - No leak
1 - Tech costs are multiplied by
    (num_players-civs_with_tech)/num_players
2 - Same as 1, but civs_with_tech is only counted from players you
    have embassy with

AI is modified to use new style of calculation.

Changes to v12:
- num_unknown_tech_reqs_rec was effectively same as
  find_prerequisites in ai/aitech.c. Changed name to
  find_unknown_tech_reqs and modified operation to match
  find_prerequisites. Removed find_prerequisites and changed AI
  to use find_unknown_tech_reqs.

I think this is ready for cvs, but please confirm this by reviewing.
Also we need comments on what to set as a default styles on default
ruleset.

--
// Juha Litola

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