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/rand.c

@@ -46,7 +46,7 @@
 #include "curl_memory.h"
 #include "memdebug.h"
 
-#ifdef WIN32
+#ifdef _WIN32
 
 #if defined(_WIN32_WINNT) && _WIN32_WINNT >= 0x600
 #  define HAVE_WIN_BCRYPTGENRANDOM
@@ -133,7 +133,7 @@ static CURLcode randit(struct Curl_easy *data, unsigned int *rnd)
 
   /* ---- non-cryptographic version following ---- */
 
-#ifdef WIN32
+#ifdef _WIN32
   if(!seeded) {
     result = Curl_win32_random((unsigned char *)rnd, sizeof(*rnd));
     if(result != CURLE_NOT_BUILT_IN)
@@ -148,7 +148,7 @@ static CURLcode randit(struct Curl_easy *data, unsigned int *rnd)
   }
 #endif
 
-#if defined(RANDOM_FILE) && !defined(WIN32)
+#if defined(RANDOM_FILE) && !defined(_WIN32)
   if(!seeded) {
     /* if there's a random file to read a seed from, use it */
     int fd = open(RANDOM_FILE, O_RDONLY);