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/digraph.c b/src/digraph.c
index 0b78af7..6b5a5aa 100644
--- a/src/digraph.c
+++ b/src/digraph.c
@@ -321,7 +321,7 @@
 	};
 
 #  else
-#   if defined(MACOS) && !defined(FEAT_MBYTE)
+#   if defined(MACOS_X) && !defined(FEAT_MBYTE)
 
 	/*
 	 * Macintosh digraphs
@@ -449,7 +449,7 @@
 	{NUL, NUL, NUL}
 	};
 
-#   else	/* !MACOS */
+#   else	/* !MACOS_X */
 
 #    ifdef OLD_DIGRAPHS