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/edit.c b/src/edit.c
index a179620..435e1ed 100644
--- a/src/edit.c
+++ b/src/edit.c
@@ -5977,7 +5977,7 @@
      * Only use mod_mask for special keys, to avoid things like <S-Space>,
      * unless 'allow_modmask' is TRUE.
      */
-#ifdef MACOS
+#ifdef MACOS_X
     /* Command-key never produces a normal key */
     if (mod_mask & MOD_MASK_CMD)
 	allow_modmask = TRUE;
@@ -8756,7 +8756,7 @@
 	    case K_KPAGEUP:
 	    case K_PAGEDOWN:
 	    case K_KPAGEDOWN:
-# ifdef MACOS
+# ifdef MACOS_X
 	    case K_LEFT:
 	    case K_RIGHT:
 	    case K_UP: