Saving blockair/blockairh is nice because RecalculateBlockAirMaps uses the sim rng, which means the sim rng would get advanced in Simulation::Load. Also rename RecalculateBlockAirMaps to ApproximateBlockAirMaps because that's what it is, an approximation, and it's needed only if there are no block air maps in the save.
Simulation::frameCount keeps track of frames elapsed since the beginning of the simulation, zeroed at clear_sim. It overflows when it reaches the 64-bit limit, which means anything that depends on it should either handle this, or not fail catastrophically. sandcolour (the only thing that depends on it as of now) is a good example of the latter: sandcolour has a periodicity of 360 frames, which means that there is one sandcolour period that is cut short by the overflow. This is not "handled" (the period is cut short, which is detectable by users) but is not catastrophic either (it's not a big deal, and it won't ever happen unless someone hacks the save).
Also restrict saves with determinism data to 98.0.
Namely: no, yes, and yes and ask at startup.
The install_check option is thus replaced by the can_install option. -Dinstall_check=true maps to -Dcan_install=yes_check, while -Dinstall_check=false maps to -Dcan_install=yes. -Dcan_install=no is new and is recommended for downstream packaging, where -Dinstall_check=false was historically used.
Also improve error messages about bad configuration here and there and scatter configuration code in subdirectories, where they can be closer to their areas of effect.
