Codechange: Redefine some cargo-related values in terms of CargoID instead of CargoType.

Values used as special filter types are now defined as offsets from NUM_CARGO instead of confusingly reusing CARGO_NO_REFIT/CARGO_AUTO_REFIT types.

Add: company mode enforcement checks to functions with command or company access

Command functions are those that call ScriptObject::Command, and functions
with company access are any that call ScriptObject::GetCompany. This is a bit
over-protective, but having the check everywhere makes it easier to validate
that no check is missing automatically instead of by review.

Fix: Some Script::IsValidVehicle checks need to be complemented with IsPrimaryVehicle

Add: [Script] ScriptVehicle.IsPrimaryVehicle

Codechange: add non-nullptr asserts in cases where it should never be nullptr

Though where similar calls are checked for nullptr as in those instances of
the use of that function it can actually return nullptr. In other words, write
down the assumption that the function never returns nullptr in an assert.

Fix #7593: Crash in ScriptOrder::GetOrderDistance in VT_AIR mode

Null pointer dereference occurred when either origin_tile or dest_tile
were waypoint tiles.