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

scripts/checksrc.pl

@@ -59,6 +59,7 @@ my %warnings = (
     'ASTERISKSPACE'    => 'pointer declared with space after asterisk',
     'BADCOMMAND'       => 'bad !checksrc! instruction',
     'BANNEDFUNC'       => 'a banned function was used',
+    'BANNEDPREPROC'    => 'a banned symbol was used on a preprocessor line',
     'BRACEELSE'        => '} else on the same line',
     'BRACEPOS'         => 'wrong position for an open brace',
     'BRACEWHILE'       => 'A single space between open brace and while',
@@ -892,6 +893,18 @@ sub scanfile {
                       "multiple spaces");
         }
       preproc:
+        if($prep) {
+          # scan for use of banned symbols on a preprocessor line
+          if($l =~ /^(^|.*\W)
+                     (WIN32)
+                     (\W|$)
+                   /x) {
+              checkwarn("BANNEDPREPROC",
+                        $line, length($1), $file, $ol,
+                        "use of $2 is banned from preprocessor lines" .
+                        (($2 eq "WIN32") ? ", use _WIN32 instead" : ""));
+          }
+        }
         $line++;
         $prevp = $prep;
         $prevl = $ol if(!$prep);