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/proto.h b/src/proto.h
index 92e3f60..a60d3c8 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -213,7 +213,7 @@
 # endif
 
 # if defined(FEAT_GUI) || defined(FEAT_JOB_CHANNEL)
-#  if defined(UNIX) || defined(MACOS)
+#  if defined(UNIX) || defined(MACOS_X)
 #   include "pty.pro"
 #  endif
 # endif
@@ -286,7 +286,7 @@
 #ifdef MACOS_CONVERT
 # include "os_mac_conv.pro"
 #endif
-#if defined(MACOS_X_UNIX) && defined(FEAT_CLIPBOARD) && !defined(FEAT_GUI)
+#if defined(MACOS_X_DARWIN) && defined(FEAT_CLIPBOARD) && !defined(FEAT_GUI)
 /* functions in os_macosx.m */
 void clip_mch_lose_selection(VimClipboard *cbd);
 int clip_mch_own_selection(VimClipboard *cbd);