Minor clang-tidy fixes

[igrr]

Hi @tomghuang, thank you for maintaining this excellent library!

This small PR fixes two issues reported in esp-idf project by clang-tidy:

1. String literals were assigned to char* variables instead of const char*. The fix looks pretty straightforward.
2. Function argument name in declaration and definition of arg_end function was different (maxerrors vs maxcount). I opted to change the name in the header file for smaller number of changes. Let me know if you prefer it the other way around.

[tomghuang]

Hi @igrr , thank you very much for this PR, which really helps to improve the quality of the project. I will try to add -Wwrite-strings to the CMake script. Thanks.

[igrr]

Thanks for the quick review and merge!

By the way, if you still have interest in #70 I can open a PR for it.

[tomghuang]

Hi @igrr ,

Yes, I'm very interested in using tools to scan security vulnerabilities in the code base. If you can provide help on #70, that would be great! Thanks!