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/cf-socket.c

@@ -81,7 +81,7 @@
 #include "memdebug.h"
 
 
-#if defined(ENABLE_IPV6) && defined(IPV6_V6ONLY) && defined(WIN32)
+#if defined(ENABLE_IPV6) && defined(IPV6_V6ONLY) && defined(_WIN32)
 /* It makes support for IPv4-mapped IPv6 addresses.
  * Linux kernel, NetBSD, FreeBSD and Darwin: default is off;
  * Windows Vista and later: default is on;
@@ -659,7 +659,7 @@ static bool verifyconnect(curl_socket_t sockfd, int *error)
   int err = 0;
   curl_socklen_t errSize = sizeof(err);
 
-#ifdef WIN32
+#ifdef _WIN32
   /*
    * In October 2003 we effectively nullified this function on Windows due to
    * problems with it using all CPU in multi-threaded cases.