patch 8.1.1891: functions used in one file are global

Problem:    Functions used in one file are global.
Solution:   Add "static". (Yegappan Lakshmanan, closes #4840)
diff --git a/src/window.c b/src/window.c
index f4154e7..fabd196 100644
--- a/src/window.c
+++ b/src/window.c
@@ -39,8 +39,11 @@
 static void enter_tabpage(tabpage_T *tp, buf_T *old_curbuf, int trigger_enter_autocmds, int trigger_leave_autocmds);
 static void frame_fix_height(win_T *wp);
 static int frame_minheight(frame_T *topfrp, win_T *next_curwin);
+static int may_open_tabpage(void);
 static void win_enter_ext(win_T *wp, int undo_sync, int no_curwin, int trigger_new_autocmds, int trigger_enter_autocmds, int trigger_leave_autocmds);
 static void win_free(win_T *wp, tabpage_T *tp);
+static int win_unlisted(win_T *wp);
+static void win_append(win_T *after, win_T *wp);
 static void frame_append(frame_T *after, frame_T *frp);
 static void frame_insert(frame_T *before, frame_T *frp);
 static void frame_remove(frame_T *frp);
@@ -3541,17 +3544,7 @@
 	emsg(_("E445: Other window contains changes"));
 }
 
-/*
- * Init the current window "curwin".
- * Called when a new file is being edited.
- */
-    void
-curwin_init(void)
-{
-    win_init_empty(curwin);
-}
-
-    void
+    static void
 win_init_empty(win_T *wp)
 {
     redraw_win_later(wp, NOT_VALID);
@@ -3574,6 +3567,16 @@
 }
 
 /*
+ * Init the current window "curwin".
+ * Called when a new file is being edited.
+ */
+    void
+curwin_init(void)
+{
+    win_init_empty(curwin);
+}
+
+/*
  * Allocate the first window and put an empty buffer in it.
  * Called from main().
  * Return FAIL when something goes wrong (out of memory).
@@ -3861,7 +3864,7 @@
  * like with ":split".
  * Returns OK if a new tab page was created, FAIL otherwise.
  */
-    int
+    static int
 may_open_tabpage(void)
 {
     int		n = (cmdmod.tab == 0) ? postponed_split_tab : cmdmod.tab;
@@ -4952,7 +4955,7 @@
  * Return TRUE if "wp" is not in the list of windows: the autocmd window or a
  * popup window.
  */
-    int
+    static int
 win_unlisted(win_T *wp)
 {
     return wp == aucmd_win || WIN_IS_POPUP(wp);
@@ -4982,7 +4985,7 @@
 /*
  * Append window "wp" in the window list after window "after".
  */
-    void
+    static void
 win_append(win_T *after, win_T *wp)
 {
     win_T	*before;