Also repurpose existing build-time feature checks into unit test 1961,
to verify.
Prior to this patch these functions were auto-detected with both
autotools and cmake. In case of autotools there was an extra
verification phase ensuring the functions work as expected. This step
required running the function, thus was limited to non-cross-builds. For
cross-builds and CMake it always used the system implementation if
present. On Windows it always used the local implementation, because
availability/use is complicated there.
After this patch all platforms, always use the local implementation,
which is known to be accurate. This makes curl behave more consistently,
and simplifies the build process, a fixes cross-builds and CMake
auto-detection differences.
Also:
- test1960: enable unconditionally.
- checksrc: disallow globally, allowlist in `block_ip.c` example.
- dnsd: verify ntop result for NULL before passing to printf.
Ref: https://github.com/curl/curl/pull/22137#issuecomment-4797440983
Ref: #22137
Ref: 8537a5b0bc#16577Closes#22170
