build: always use local inet_pton()/inet_ntop() implementations

[vszakats]

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: #22137 (comment)
Ref: #22137
Ref: 8537a5b #16577

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• verify if there is more now unused build logic that can be deleted.
  (such as including headers that are now unused)
  possibly separate PR.
• possibly streamline these APIs to return error without relying on errno,
  and passing a socket error through it.
  → Check if errno is used at all. [YES for inet_ntop only: cf-socket.c / sockaddr2string() does use it]
  → curlx_inet_ntop: return socket error via argument #22229
• possibly streamline these APIs to accept locally defined constants
  to select IPv4 vs IPv6. That'll also need untangling it from AF_INET[6]
  in some callers that pass through that value from structs/variables.
  It'd be useful because IPv6 support is also maintained here in builds/envs
  with IPv6 support missing/disabled, and in such case there is local hack
  adding a surrogate AF_INET6 macro, with fragile detection, and stomping on
  a system symbol.
  → figure out which headers define it, and perhaps introduce a local macro
  on top of external ones?

[Copilot]

Pull request overview

This PR standardizes curl’s behavior across build systems and platforms by always using curl’s local curlx_inet_pton() / curlx_inet_ntop() implementations, removing the prior autotools/CMake feature-detection (and runtime verification) logic that could diverge—especially for cross-builds.

Changes:

• Remove autotools and CMake detection/verification of system inet_pton() / inet_ntop() and related config defines.
• Add unit test coverage (unit1961 + test1961) to verify curlx_inet_pton() / curlx_inet_ntop() behavior.
• Enforce inet_pton() / inet_ntop() as banned functions via checksrc, with an explicit allowlist in the block_ip.c example.

Reviewed changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/unit/unit1961.c	Adds a new unit test validating curlx_inet_ntop() / curlx_inet_pton() formatting/parsing behavior.
tests/unit/Makefile.inc	Registers the new unit test source in the unit test build.
tests/server/dnsd.c	Avoids logging NULL from curlx_inet_ntop() by handling failures explicitly.
tests/libtest/Makefile.inc	Ensures curlx inet implementations are linked into libtests where needed (notably for shared builds).
tests/libtest/lib1960.c	Switches test1960 to always use curlx_inet_pton() and removes conditional compilation around system availability.
tests/libtest/first.h	Includes curlx/inet_ntop.h and curlx/inet_pton.h for libtest compilation.
tests/data/test1961	Adds a new test definition for running unit1961 via the test harness.
tests/data/Makefile.am	Adds test1961 to the distributed test data list.
scripts/checksrc.pl	Bans inet_pton / inet_ntop usage across the tree via checksrc.
m4/curl-functions.m4	Removes autotools include helper and the inet_pton / inet_ntop detection + runtime verification macros.
lib/curlx/inet_pton.h	Removes system-function macro mapping; always declares curlx_inet_pton().
lib/curlx/inet_pton.c	Builds the local curlx_inet_pton() implementation unconditionally (no longer gated on HAVE_INET_PTON).
lib/curlx/inet_ntop.h	Removes system-function macro mapping; always declares curlx_inet_ntop().
lib/curlx/inet_ntop.c	Builds the local curlx_inet_ntop() implementation unconditionally (no longer gated on HAVE_INET_NTOP).
lib/curl_config-cmake.h.in	Removes HAVE_INET_PTON / HAVE_INET_NTOP config defines from the CMake config header template.
lib/config-win32.h	Removes stale comments/undefs related to HAVE_INET_PTON / HAVE_INET_NTOP.
docs/internals/CODE_STYLE.md	Documents inet_pton / inet_ntop as banned functions.
docs/examples/block_ip.c	Adds checksrc allowlist directives for intentional system inet_pton / inet_ntop usage in the example.
configure.ac	Stops invoking the autotools checks for system inet_pton / inet_ntop.
CMakeLists.txt	Stops probing for system inet_pton / inet_ntop in non-Windows builds.
CMake/win32-cache.cmake	Removes cached HAVE_INET_* entries that are no longer used.
CMake/unix-cache.cmake	Removes cached HAVE_INET_* entries that are no longer used.

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[vszakats]

(Pre-existing / unrelated.)