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/keymap.h b/src/keymap.h
index 52ba319..7cb5c69 100644
--- a/src/keymap.h
+++ b/src/keymap.h
@@ -479,7 +479,7 @@
 #define MOD_MASK_2CLICK	    0x20	/* use MOD_MASK_MULTI_CLICK */
 #define MOD_MASK_3CLICK	    0x40	/* use MOD_MASK_MULTI_CLICK */
 #define MOD_MASK_4CLICK	    0x60	/* use MOD_MASK_MULTI_CLICK */
-#ifdef MACOS
+#ifdef MACOS_X
 # define MOD_MASK_CMD	    0x80
 #endif