[WIP] Use nullptr in wincurse.cpp where appropriate

[luluvia]

Still needs some more work to ensure that this update works across different platforms and compilers. Not ready for review. May need to split into another pull request for a separate fix that is currently included in this.

Summary

SUMMARY: None

Purpose of change

Compiling with GCC 9 through CMake with MSYS 2 on Windows 10 throws warnings which are treated as errors regarding zeros being used as null pointers. Important to note these warnings only seem to appear through CMake, not make.

Describe the solution

This change replaces zeros with nullptr where appropriate, removes the unused macro, and simplifies a couple of "if" statements. It also resolves an issue where an error is thrown only by CMake compiling for Windows regarding the _UNICODE define.

Describe alternatives you've considered

I've tried compiling with Ubuntu 18.04 (using g++ 7 and then g++ 9 in separate tests) and it seems to be working as normal.

We could just not update those portions, but I believe it's not a bad idea to ensure these warnings get cleaned up.

Testing

Since this involves wincurse.cpp, build for curses and launch. Try using different features like options or load into the world. Close the game. There shouldn't be any errors in these displays.

Buildlog (ommitted user directories)

[Qrox]

I believe there was plan to obsolete the wincurse build? @ZhilkinSerg

[ZhilkinSerg]

Yeah, but in the meantime small fixes are okay.