Freeciv

I haven't thought about this, but linking to S3_2-d3f so that it will be considered in time for 3.2.

Well, after several iterations (internal to my own my own development machine) it's better to just ignore earlier comments, and take a fresh look at this implementation.

- Unhomed units have their gold upkeep based on the same effects as homed ones. The main change this patch brings is that when previously units were iterated for upkeep from the list of their supporting city, now the iteration is over the list of player's owned units, and thus including homeless units (not in the list of any city)
- This is functionality is never available when gold_upkeep_style is "City" for obvious conflict between the settings (can't have city to pay upkeep for units that have no city)
- In this initial patch compatibility mode for older rules is controlled simply by global boolean "homeless_gold_upkeep" ruleset setting. Old behavior is kept as the default for now. I hope that better mechanisms for these emerge during 3.2 cycle already. I think it's better to have this in its current form already, than to wait that dependencies of better mechanisms get implemented (mainly meaning that requirements checking unit homelessness should work for all unit upkeep effects)