cmake: restrict static CRT builds to static curl exe, test in CI

Static CRT crashes MSVCR* MSVC builds (in VS2008, VS2010, VS2012,
VS2013) according to CI and local tests. The reproducible crash happens
in `curl_mfprintf() -> fputc(s, stderr)` when trying to display the
warning message in `curl -V`. `stderr` is non-NULL and resolves to `2`.
This reproducer needs a debug-enabled build, but it's unrelated to debug
features or curl's memory tracker. It happens regardless of unity build,
CPU architecture or `DllMain()` use. Example from VS2013:

```
+ _bld/src/Debug/curl.exe --disable --version
./appveyor.sh: line 124:   203 Segmentation fault      "${curl}" --disable --version
```
Ref: https://ci.appveyor.com/project/curlorg/curl/builds/51570451/job/ojpdqrsm1hmpmq6a#L210

Another crash happened in an UCRT build (VS2017) with a couple of
`printf()`s added to curl's `main()` function:

```
Microsoft Visual C++ Runtime Library
Debug Assertion Failed!
Program: C:/projects/curl/bld/src/Debug/curl.exe
File: minkernel/crts/ucrt/src/appcrt/heap/debug_heap.cpp
Line: 996
Expression: _act_first_block == header
```
(it hangs the job in CI due to the GUI popup)
Ref: https://github.com/curl/curl/pull/16394#issuecomment-2677181716

To avoid actual and potential issues, this patch issues a warning on
the shared-libcurl + static-CRT combination and falls back to the
default, shared CRT. IOW a static CRT build now requires a static curl
exe when using the `CURL_STATIC_CRT=ON` option.

Follow-up to 4fc6ebe18a #1621
Cherry-picked from #16394 (with more details there)

Closes #16456

appveyor.sh

@@ -58,6 +58,7 @@ if [ "${BUILD_SYSTEM}" = 'CMake' ]; then
       -DCMAKE_UNITY_BUILD="${UNITY}" -DCURL_TEST_BUNDLES=ON \
       -DCURL_WERROR=ON \
       -DBUILD_SHARED_LIBS="${SHARED}" \
+      -DCURL_STATIC_CRT=ON \
       -DENABLE_DEBUG="${DEBUG}" \
       -DENABLE_UNICODE="${ENABLE_UNICODE}" \
       -DHTTP_ONLY="${HTTP_ONLY}" \