places on Windows: fix OS thread identification

The bug particularly broke `#:async-apply` handling for an FFI callback,
causing the current thread always to be equated with the target thread.
For example, the teachpack documentation (which now renders images
to SVG) kept crashing on a multi-place build due to callbacks getting
invoked in the wrong place.

racket/src/racket/src/mzrt.c

@@ -187,7 +187,7 @@ DWORD WINAPI mzrt_win_thread_stub(void *data)
 
 mzrt_thread_id mz_proc_thread_self() {
 #ifdef WIN32
-  return GetCurrentThread();
+  return GetCurrentThreadId();
 #else
   return pthread_self();
 #endif

racket/src/racket/src/mzrt.h

@@ -21,7 +21,7 @@ void mzrt_set_user_break_handler(void (*user_break_handler)(int));
 
 #if (defined(__WIN32__) || defined(WIN32) || defined(_WIN32))
 # include <winsock2.h>
-typedef HANDLE mzrt_thread_id;
+typedef DWORD mzrt_thread_id;
 #else
 #include <pthread.h>
 typedef pthread_t mzrt_thread_id;