Clean up __OFSUB__ usage

[AJenbo]

This removes remaining usage of __OFSUB__, this is basically continuation of bb3ab09

This enables us to compile the following files as C:
drlg_l3.cpp
lighting.cpp
monster.cpp
stores.cpp

I have play tested things and haven't seen any negative sides to doing this.

[mewmew]

My 3am brain can't quite determine whether the translation here is correct and covers all cases (e.g handling of edge cases, int overflow, underflow). I would therefore like to refrain from merging until this has been validated. Even if it passes the it compiles test and the playable test :)

Oh, and happy new year @AJenbo!

[AJenbo]

Yeah get some sleep and then you can compare with #538 and #536 where I fully cleaned up some functions that makes use of __OFSUB__ :)

[AJenbo]

Just as a note the reason we feel it's safe to remove __OFSUB__ is that it appears to simply be a side effect of how the compiler optimized if (a < b) into if (a - b < 0) in some cases, and as such the extra edge case protection that __OFSUB__ gives isn't needed.