tests/server: round of tidy-ups (part 2)

tests/server: round of tidy-ups (part 2)

General tidy-ups, to identify and reduce duplications and potential
issues, while also making the server modules compile as a single binary.

- ensure unique symbols and no shadowing across server sources, by
  renaming variables.
- move globals common to multiple servers into shared `util` module.
- drop constants with a single use.
- undef macro before re-using them across server sources.
- move common functions into shared `util` module.
- drop redundant static declarations.
- disable IPv6 code when built without IPv6.
- start syncing the 3 almost identical copies of `sockdaemon` function.
- drop unused `timeval.h` header.
- drop `poll()` from `wait_ms()`, for macOS, following an earlier core
  update.
  Follow-up to c72cefea0fadaf4114a0036c86005ee5739ec30a #15096

Follow-up to 9213e4e497d575d2bc2c9265d40da6c5549f526d #16525
Cherry-picked from #15000
Closes #16609