Remove configure probe for fdatasync.

fdatasync() is in SUSv2, and all targeted Unix systems have it.  We have
a replacement function for Windows.

We retain the probe for the function declaration, which allows us to
supply the mysteriously missing declaration for macOS, and also for
Windows.  No need to keep a HAVE_FDATASYNC macro around.

Also rename src/port/fdatasync.c to win32fdatasync.c since it's only for
Windows.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA+hUKGJ3LHeP9w5Fgzdr4G8AnEtJ=z=p6hGDEm4qYGEUX5B6fQ@mail.gmail.com
Discussion: https://postgr.es/m/CA%2BhUKGJZJVO%3DiX%2Beb-PXi2_XS9ZRqnn_4URh0NUQOwt6-_51xQ%40mail.gmail.com

src/port/win32fdatasync.c

@@ -0,0 +1,53 @@
+/*-------------------------------------------------------------------------
+ *
+ * win32fdatasync.c
+ *	   Win32 fdatasync() replacement
+ *
+ *
+ * Portions Copyright (c) 1996-2022, PostgreSQL Global Development Group
+ *
+ * src/port/win32fdatasync.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#define UMDF_USING_NTSTATUS
+
+#ifdef FRONTEND
+#include "postgres_fe.h"
+#else
+#include "postgres.h"
+#endif
+
+#include "port/win32ntdll.h"
+
+int
+fdatasync(int fd)
+{
+	IO_STATUS_BLOCK iosb;
+	NTSTATUS	status;
+	HANDLE		handle;
+
+	handle = (HANDLE) _get_osfhandle(fd);
+	if (handle == INVALID_HANDLE_VALUE)
+	{
+		errno = EBADF;
+		return -1;
+	}
+
+	if (initialize_ntdll() < 0)
+		return -1;
+
+	memset(&iosb, 0, sizeof(iosb));
+	status = pg_NtFlushBuffersFileEx(handle,
+									 FLUSH_FLAGS_FILE_DATA_SYNC_ONLY,
+									 NULL,
+									 0,
+									 &iosb);
+
+	if (NT_SUCCESS(status))
+		return 0;
+
+	_dosmaperr(pg_RtlNtStatusToDosError(status));
+	return -1;
+}