patch 9.0.0965: using one window for executing autocommands is insufficient

Problem:    Using one window for executing autocommands is insufficient.
Solution:   Use up to five windows for executing autocommands.
diff --git a/src/structs.h b/src/structs.h
index f57fb85..824de45 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -4160,7 +4160,7 @@
 typedef struct
 {
     buf_T	*save_curbuf;	    // saved curbuf
-    int		use_aucmd_win;	    // using aucmd_win
+    int		use_aucmd_win_idx;  // index in aucmd_win[] if >= 0
     int		save_curwin_id;	    // ID of saved curwin
     int		new_curwin_id;	    // ID of new curwin
     int		save_prevwin_id;    // ID of saved prevwin