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/socketpair.c

@@ -28,7 +28,7 @@
 #include "rand.h"
 
 #if !defined(HAVE_SOCKETPAIR) && !defined(CURL_DISABLE_SOCKETPAIR)
-#ifdef WIN32
+#ifdef _WIN32
 /*
  * This is a socketpair() implementation for Windows.
  */
@@ -50,7 +50,7 @@
 #ifndef INADDR_LOOPBACK
 #define INADDR_LOOPBACK 0x7f000001
 #endif /* !INADDR_LOOPBACK */
-#endif /* !WIN32 */
+#endif /* !_WIN32 */
 
 #include "nonblock.h" /* for curlx_nonblock */
 #include "timeval.h"  /* needed before select.h */
@@ -87,7 +87,7 @@ int Curl_socketpair(int domain, int type, int protocol,
 
   socks[0] = socks[1] = CURL_SOCKET_BAD;
 
-#if defined(WIN32) || defined(__CYGWIN__)
+#if defined(_WIN32) || defined(__CYGWIN__)
   /* don't set SO_REUSEADDR on Windows */
   (void)reuse;
 #ifdef SO_EXCLUSIVEADDRUSE