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/memdebug.h

@@ -64,7 +64,7 @@ CURL_EXTERN ALLOC_SIZE(2) void *curl_dbg_realloc(void *ptr,
 CURL_EXTERN void curl_dbg_free(void *ptr, int line, const char *source);
 CURL_EXTERN ALLOC_FUNC char *curl_dbg_strdup(const char *str, int line,
                                              const char *src);
-#if defined(WIN32) && defined(UNICODE)
+#if defined(_WIN32) && defined(UNICODE)
 CURL_EXTERN ALLOC_FUNC wchar_t *curl_dbg_wcsdup(const wchar_t *str,
                                                 int line,
                                                 const char *source);
@@ -121,7 +121,7 @@ CURL_EXTERN int curl_dbg_fclose(FILE *file, int line, const char *source);
 #define send(a,b,c,d) curl_dbg_send(a,b,c,d, __LINE__, __FILE__)
 #define recv(a,b,c,d) curl_dbg_recv(a,b,c,d, __LINE__, __FILE__)
 
-#ifdef WIN32
+#ifdef _WIN32
 #  ifdef UNICODE
 #    undef wcsdup
 #    define wcsdup(ptr) curl_dbg_wcsdup(ptr, __LINE__, __FILE__)