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

src/tool_util.c

@@ -29,6 +29,7 @@
 
 #include "tool_util.h"
 
+#include "curlx.h"
 #include "memdebug.h" /* keep this as LAST include */
 
 #if defined(_WIN32)
@@ -188,3 +189,33 @@ int tool_ftruncate64(int fd, curl_off_t where)
 }
 
 #endif /* USE_TOOL_FTRUNCATE */
+
+#ifdef _WIN32
+FILE *Curl_execpath(const char *filename, char **pathp)
+{
+  static char filebuffer[512];
+  unsigned long len;
+  /* Get the filename of our executable. GetModuleFileName is already declared
+   * via inclusions done in setup header file. We assume that we are using
+   * the ASCII version here.
+   */
+  len = GetModuleFileNameA(0, filebuffer, sizeof(filebuffer));
+  if(len > 0 && len < sizeof(filebuffer)) {
+    /* We got a valid filename - get the directory part */
+    char *lastdirchar = strrchr(filebuffer, DIR_CHAR[0]);
+    if(lastdirchar) {
+      size_t remaining;
+      *lastdirchar = 0;
+      /* If we have enough space, build the RC filename */
+      remaining = sizeof(filebuffer) - strlen(filebuffer);
+      if(strlen(filename) < remaining - 1) {
+        msnprintf(lastdirchar, remaining, "%s%s", DIR_CHAR, filename);
+        *pathp = filebuffer;
+        return fopen(filebuffer, FOPEN_READTEXT);
+      }
+    }
+  }
+
+  return NULL;
+}
+#endif