fcdb: create_gameid()
Ended with a bit different implementation. Most notably the function is called "game_start", and it's called when game actually starts. This dbid is also stored to savegame. On game restart server proposes dbid from the savegame to the lua interface, which then can either use it or to create a new one.
New version:
- Really pass existing dbid to database.lua
- Fixed integer type problems
- Pass '-1' as "no dbid" value, consider zero valid value
Query game id from a fcdb lua script. The main use-case is that the fcdb database can initialise game what ever way it sees fit, and to just give freeciv code an id by which it itself knows the game. Later on freeciv code can pass the id back to any database query functions.