Fix compiling with Clang 18 and a few warnings

[jmcarcell]

The header is needed to be able to find ::open and ::close. For the warnings I commented out one case instead of removing it because the place where it's used is also commented out (here, so it would be easy to reenable in case someone wanted to...).

BEGINRELEASENOTES

• Add the missing header unistd.h in a file to find ::open and ::close
• Fix a few warnings about unused variables or shadowing
• Add .cache and compile_commands.json to .gitignore

ENDRELEASENOTES

[github-actions]

Test Results

   14 files     14 suites   6h 39m 28s ⏱️
  365 tests   365 ✅ 0 💤 0 ❌
2 510 runs  2 510 ✅ 0 💤 0 ❌

Results for commit e3264a5.