windows: use built-in _WIN32 macro to detect Windows

Windows compilers define `_WIN32` automatically. Windows SDK headers
or build env defines `WIN32`, or we have to take care of it. The
agreement seems to be that `_WIN32` is the preferred practice here.
Make the source code rely on that to detect we're building for Windows.

Public `curl.h` was using `WIN32`, `__WIN32__` and `CURL_WIN32` for
Windows detection, next to the official `_WIN32`. After this patch it
only uses `_WIN32` for this. Also, make it stop defining `CURL_WIN32`.

There is a slight chance these break compatibility with Windows
compilers that fail to define `_WIN32`. I'm not aware of any obsolete
or modern compiler affected, but in case there is one, one possible
solution is to define this macro manually.

grepping for `WIN32` remains useful to discover Windows-specific code.

Also:

- extend `checksrc` to ensure we're not using `WIN32` anymore.

- apply minor formatting here and there.

- delete unnecessary checks for `!MSDOS` when `_WIN32` is present.

Co-authored-by: Jay Satiro
Reviewed-by: Daniel Stenberg

Closes #12376

lib/strerror.h

@@ -29,7 +29,7 @@
 #define STRERROR_LEN 256 /* a suitable length */
 
 const char *Curl_strerror(int err, char *buf, size_t buflen);
-#if defined(WIN32) || defined(_WIN32_WCE)
+#if defined(_WIN32) || defined(_WIN32_WCE)
 const char *Curl_winapi_strerror(DWORD err, char *buf, size_t buflen);
 #endif
 #ifdef USE_WINDOWS_SSPI