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

docs/examples/10-at-a-time.c

@@ -29,7 +29,7 @@
 #include <errno.h>
 #include <stdlib.h>
 #include <string.h>
-#ifndef WIN32
+#ifndef _WIN32
 #  include <unistd.h>
 #endif
 #include <curl/curl.h>

docs/examples/anyauthput.c

@@ -33,7 +33,7 @@
 
 #include <curl/curl.h>
 
-#ifdef WIN32
+#ifdef _WIN32
 #  define FILENO(fp) _fileno(fp)
 #else
 #  define FILENO(fp) fileno(fp)

docs/examples/cookie_interface.c

@@ -91,7 +91,7 @@ main(void)
 
     printf("-----------------------------------------------\n"
            "Setting a cookie \"PREF\" via cookie interface:\n");
-#ifdef WIN32
+#ifdef _WIN32
 #define snprintf _snprintf
 #endif
     /* Netscape format cookie */

docs/examples/externalsocket.c

@@ -30,7 +30,7 @@
 #include <stdlib.h>
 #include <curl/curl.h>
 
-#ifdef WIN32
+#ifdef _WIN32
 #include <windows.h>
 #include <winsock2.h>
 #include <ws2tcpip.h>
@@ -96,7 +96,7 @@ int main(void)
   struct sockaddr_in servaddr;  /*  socket address structure  */
   curl_socket_t sockfd;
 
-#ifdef WIN32
+#ifdef _WIN32
   WSADATA wsaData;
   int initwsa = WSAStartup(MAKEWORD(2, 2), &wsaData);
   if(initwsa) {
@@ -168,7 +168,7 @@ int main(void)
     }
   }
 
-#ifdef WIN32
+#ifdef _WIN32
   WSACleanup();
 #endif
   return 0;

docs/examples/ftpupload.c

@@ -29,7 +29,7 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <errno.h>
-#ifdef WIN32
+#ifdef _WIN32
 #include <io.h>
 #else
 #include <unistd.h>

docs/examples/ipv6.c

@@ -28,13 +28,13 @@
 #include <stdio.h>
 #include <curl/curl.h>
 
-#ifndef WIN32
+#ifndef _WIN32
 #include <net/if.h>
 #endif
 
 int main(void)
 {
-#ifndef WIN32
+#ifndef _WIN32
   /* Windows users need to find how to use if_nametoindex() */
   CURL *curl;
   CURLcode res;