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/getchar.c b/src/getchar.c
index ef42d9c..455c013 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -1893,7 +1893,7 @@
     int	    retval;
 
 #ifdef FEAT_EVAL
-    /* When test_disable_char_avail(1) was called pretend there is no
+    /* When test_override("char_avail", 1) was called pretend there is no
      * typeahead. */
     if (disable_char_avail_for_testing)
 	return FALSE;
@@ -5257,7 +5257,7 @@
 }
 #endif
 
-#if defined(MSWIN) || defined(MACOS)
+#if defined(MSWIN) || defined(MACOS_X)
 
 #define VIS_SEL	(VISUAL+SELECTMODE)	/* abbreviation */
 
@@ -5308,7 +5308,7 @@
 # endif
 #endif
 
-#if defined(MACOS)
+#if defined(MACOS_X)
 	/* Use the Standard MacOS binding. */
 	/* paste, copy and cut */
 	{(char_u *)"<D-v> \"*P", NORMAL},
@@ -5329,7 +5329,7 @@
     void
 init_mappings(void)
 {
-#if defined(MSWIN) ||defined(MACOS)
+#if defined(MSWIN) || defined(MACOS_X)
     int		i;
 
     for (i = 0; i < (int)(sizeof(initmappings) / sizeof(struct initmap)); ++i)
@@ -5337,7 +5337,8 @@
 #endif
 }
 
-#if defined(MSWIN) || defined(FEAT_CMDWIN) || defined(MACOS) || defined(PROTO)
+#if defined(MSWIN) || defined(FEAT_CMDWIN) || defined(MACOS_X) \
+							     || defined(PROTO)
 /*
  * Add a mapping "map" for mode "mode".
  * Need to put string in allocated memory, because do_map() will modify it.