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

include/curl/curl.h

@@ -53,15 +53,6 @@
 #include "curlver.h"         /* libcurl version defines   */
 #include "system.h"          /* determine things run-time */
 
-/*
- * Define CURL_WIN32 when build target is Win32 API
- */
-
-#if (defined(_WIN32) || defined(__WIN32__) || defined(WIN32)) &&        \
-  !defined(__SYMBIAN32__)
-#define CURL_WIN32
-#endif
-
 #include <stdio.h>
 #include <limits.h>
 
@@ -74,7 +65,7 @@
 #include <sys/types.h>
 #include <time.h>
 
-#if defined(CURL_WIN32) && !defined(_WIN32_WCE) && !defined(__CYGWIN__)
+#if defined(_WIN32) && !defined(_WIN32_WCE) && !defined(__CYGWIN__)
 #if !(defined(_WINSOCKAPI_) || defined(_WINSOCK_H) || \
       defined(__LWIP_OPT_H__) || defined(LWIP_HDR_OPT_H))
 /* The check above prevents the winsock2 inclusion if winsock.h already was
@@ -97,11 +88,11 @@
 #include <sys/select.h>
 #endif
 
-#if !defined(CURL_WIN32) && !defined(_WIN32_WCE)
+#if !defined(_WIN32) && !defined(_WIN32_WCE)
 #include <sys/socket.h>
 #endif
 
-#if !defined(CURL_WIN32)
+#if !defined(_WIN32)
 #include <sys/time.h>
 #endif
 
@@ -128,7 +119,7 @@ typedef void CURLSH;
 
 #ifdef CURL_STATICLIB
 #  define CURL_EXTERN
-#elif defined(CURL_WIN32) || defined(__SYMBIAN32__) || \
+#elif defined(_WIN32) || defined(__SYMBIAN32__) || \
      (__has_declspec_attribute(dllexport) && \
       __has_declspec_attribute(dllimport))
 #  if defined(BUILDING_LIBCURL)
@@ -144,7 +135,7 @@ typedef void CURLSH;
 
 #ifndef curl_socket_typedef
 /* socket typedef */
-#if defined(CURL_WIN32) && !defined(__LWIP_OPT_H__) && !defined(LWIP_HDR_OPT_H)
+#if defined(_WIN32) && !defined(__LWIP_OPT_H__) && !defined(LWIP_HDR_OPT_H)
 typedef SOCKET curl_socket_t;
 #define CURL_SOCKET_BAD INVALID_SOCKET
 #else