Refactor: code hygiene: move towards static well-behavedness of globals

This prevents accidental variable shadowing in the future, helps static
analyzers without cross-file (inter-unit) capabilities, and, for
instance, shall speed up static linking a tiny bit. Normally,
every global is declared static unless there's a good reason not to...

Also drop some superfluous declarations, redundant with the content of
included headers.

External symbols' stats for a particular configuration of the build[*]:

• before: 3170
• after: 2989

• find -executable -type f -cnewer configure \ -exec nm -g --defined-only {} \; 2>/dev/null | wc -l