Fix simple warnings

[GravisZro]

Most of these warnings are due to the use of NULL instead of 0 or NULL instead of '\0'. Some are macro redefinitions. None of them are pointer storage related. Those will be in another PR.

[Arcnor]

Can you use nullptr instead of \0?

[GravisZro]

Neither NULL nor nullptr are characters. '\0' is a character, so no, I cannot use nullptr.

[Arcnor]

Oh, this is assigning null terminators to strings, I see. Not sure why there are so many instances of that, though.

[winterheart]

Is there more secure way to do that with memset?

[GravisZro]

@winterheart Honestly, all the string building/manipulation code is obsolete garbage that needs to be replaced. At this point, I'm merely clearing up warning messages so that I can make a sane build before making major changes.

[Lgt2x]

I still have some implicit conversion of NULL constant to 'char' in some places, but certainly fewer. Thanks!