curl: add options for safe/no CA bundle search (Windows)

Add `CURL_CA_SEARCH_SAFE` build-time option to enable CA bundle search
in the `curl` tool directory. The lookup method was already used to find
`.curlrc` and `_curlrc` (on Windows). On Windows it overrides the unsafe
default `SearchPath()` method.

Enable with:
- cmake: `-DCURL_CA_SEARCH_SAFE=ON`
- autotools: `--enable-ca-search-safe`
- raw: `CPPFLAGS=-DCURL_CA_SEARCH_SAFE`

On Windows, before this patch the whole `PATH` was searched for
a CA bundle. `PATH` may contain unwanted or world-writable locations,
including the current directory. Searching them all is convenient to
pick up any CA bundle, but not secure.

The Muldersoft curl distro implements such CA search via a custom
patch for Windows:
cd652d4792/patch/curl_tool_doswin.diff (L50)

MSYS2/mingw-w64 distro has also been rolling a patch solving this:
https://github.com/msys2/MINGW-packages/blob/master/mingw-w64-curl/0001-Make-cURL-relocatable.patch
https://github.com/msys2/MINGW-packages/blob/master/mingw-w64-curl/pathtools.c

Also add option to fully disable Windows CA search:
- cmake: `-DCURL_DISABLE_CA_SEARCH=ON`
- autotools: `--disable-ca-search`
- raw: `CPPFLAGS=-DCURL_DISABLE_CA_SEARCH`.

Both options are considered EXPERIMENTAL, with possible incompatible
changes or even (partial) removal in the future, depending on feedback.

An alternative, secure option is to embed the CA bundle into the binary.

Safe search can be extended to other platforms if necessary or useful,
by using `_NSGetExecutablePath()` (macOS),
`/proc/self/exe` (Linux/Cygwin), or `argv[0]`.

Closes #14582

lib/curl_config.h.cmake

@@ -160,6 +160,12 @@
 /* disables verbose strings */
 #cmakedefine CURL_DISABLE_VERBOSE_STRINGS 1
 
+/* disables unsafe CA bundle search on Windows from the curl tool */
+#cmakedefine CURL_DISABLE_CA_SEARCH 1
+
+/* safe CA bundle search (within the curl tool directory) on Windows */
+#cmakedefine CURL_CA_SEARCH_SAFE 1
+
 /* to make a symbol visible */
 #cmakedefine CURL_EXTERN_SYMBOL ${CURL_EXTERN_SYMBOL}
 /* Ensure using CURL_EXTERN_SYMBOL is possible */