patch 8.1.0941: macros for MS-Windows are inconsistent

Problem:    Macros for MS-Windows are inconsistent, using "32", "3264 and
            others.
Solution:   Use MSWIN for all MS-Windows builds.  Use FEAT_GUI_MSWIN for the
            GUI build. (Hirohito Higashi, closes #3932)
diff --git a/src/ui.c b/src/ui.c
index 0823d2d..e21e801 100644
--- a/src/ui.c
+++ b/src/ui.c
@@ -40,7 +40,7 @@
     /* Don't output anything in silent mode ("ex -s") unless 'verbose' set */
     if (!(silent_mode && p_verbose == 0))
     {
-#if !defined(WIN3264)
+#if !defined(MSWIN)
 	char_u	*tofree = NULL;
 
 	if (output_conv.vc_type != CONV_NONE)
@@ -54,7 +54,7 @@
 
 	mch_write(s, len);
 
-# if !defined(WIN3264)
+# if !defined(MSWIN)
 	if (output_conv.vc_type != CONV_NONE)
 	    vim_free(tofree);
 # endif
@@ -62,7 +62,7 @@
 #endif
 }
 
-#if defined(UNIX) || defined(VMS) || defined(PROTO) || defined(WIN3264)
+#if defined(UNIX) || defined(VMS) || defined(PROTO) || defined(MSWIN)
 /*
  * When executing an external program, there may be some typed characters that
  * are not consumed by it.  Give them back to ui_inchar() and they are stored
@@ -3435,7 +3435,7 @@
 
 #endif /* FEAT_MOUSE */
 
-#if defined(FEAT_GUI) || defined(WIN3264) || defined(PROTO)
+#if defined(FEAT_GUI) || defined(MSWIN) || defined(PROTO)
 /*
  * Called when focus changed.  Used for the GUI or for systems where this can
  * be done in the console (Win32).