patch 8.0.1236: Mac features are confusing

Problem:    Mac features are confusing.
Solution:   Make feature names more consistent, add "osxdarwin". Rename
            feature flags, cleanup Mac code. (Kazunobu Kuriyama, closes #2178)
diff --git a/src/undo.c b/src/undo.c
index 792d791..2c5725f 100644
--- a/src/undo.c
+++ b/src/undo.c
@@ -1750,7 +1750,7 @@
     if (!write_ok)
 	EMSG2(_("E829: write error in undo file: %s"), file_name);
 
-#if defined(MACOS_CLASSIC) || defined(WIN3264)
+#if defined(WIN3264)
     /* Copy file attributes; for systems where this can only be done after
      * closing the file. */
     if (buf->b_ffname != NULL)