lib: include necessary headers for `inet_ntop`/`inet_pton`

lib: include necessary headers for `inet_ntop`/`inet_pton`

Include `netinet/in.h` for FreeBSD/OpenBSD. Also include `sys/socket.h`
just in case, based on earlier code in `tests/libtest/lib1960.c`.

Also:
- document these in `CMakeLists.txt`.
- add a CI job testing FreeBSD with no unity and no test bundles.
  (without running tests to keep it fast)

FreeBSD (autotools):
```
../../../tests/libtest/lib1960.c:66:22: error: variable has incomplete type 'struct sockaddr_in'
   66 |   struct sockaddr_in serv_addr;
      |                      ^
../../../tests/libtest/lib1960.c:66:10: note: forward declaration of 'struct sockaddr_in'
   66 |   struct sockaddr_in serv_addr;
      |          ^
```
Ref: https://github.com/curl/curl/actions/runs/13159721509/job/36725114118?pr=16188#step:3:5289

OpenBSD (cmake):
```
/home/runner/work/curl/curl/tests/libtest/lib1960.c:66:22: error: variable has incomplete type 'struct sockaddr_in'
  struct sockaddr_in serv_addr;
                     ^
/home/runner/work/curl/curl/tests/libtest/lib1960.c:66:10: note: forward declaration of 'struct sockaddr_in'
  struct sockaddr_in serv_addr;
         ^
1 error generated.
```
Ref: https://github.com/curl/curl/actions/runs/13159721509/job/36725102004?pr=16188#step:3:2166

Reported-by: CueXXIII on Github
Fixes #16184
Follow-up to a3585c9576abccddbd27200058912cef900c3c0f #15543
Closes #16188