Cleanup shadows variable warnings, round 1

Hi Hackers,

While reviewing [1], it makes me recall an experience where I had a patch ready locally, but CommitFest CI failed with a shadows-variable warning. Now I understand that -Wall doesn't by default enable -Wshadows with some compilers like clang.

I did a clean build with manually enabling -Wshadow and surprisingly found there are a lot of such warnings in the current code base, roughly ~200 occurrences.

As there are too many, I plan to fix them all in 3-4 rounds. For round 1 patch, I'd see any objection, then decide if to proceed more rounds.

Best regards,