Fix all luacheck warnings

[jaromil]

The Luacheck setup has improved to the point that all warnings are relevant and may point to a lack of local variable declarations or shadow values and other trivial errors in code. It makes sense to go through all of them and solve them whenever we have time (they are less than 400 at the time of writing this issue, with minor repetition).

apt-get install lua-check
luacheck --config src/lua/.luacheckrc --no-colors --codes src/lua/**.lua | less

Any check we want to deactivate has a W000 code that can be added to the .luackeckrc