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/warnless.c

@@ -37,7 +37,7 @@
 
 #include "warnless.h"
 
-#ifdef WIN32
+#ifdef _WIN32
 #undef read
 #undef write
 #endif
@@ -367,7 +367,7 @@ curl_socket_t curlx_sitosk(int i)
 
 #endif /* USE_WINSOCK */
 
-#if defined(WIN32)
+#if defined(_WIN32)
 
 ssize_t curlx_read(int fd, void *buf, size_t count)
 {
@@ -379,7 +379,7 @@ ssize_t curlx_write(int fd, const void *buf, size_t count)
   return (ssize_t)write(fd, buf, curlx_uztoui(count));
 }
 
-#endif /* WIN32 */
+#endif /* _WIN32 */
 
 /* Ensure that warnless.h redefinitions continue to have an effect
    in "unity" builds. */