terminal_mux: Clear terminal when activating menu

This patch clears the terminal each time the menu appears and thereby
wipes away artifacts that might occur when combining terminal_mux with
kdb_uart_drv and the kernel prints messages directly.

gems/src/server/terminal_mux/main.cc

@@ -610,6 +610,7 @@ void Session_manager::activate_menu()
 	_menu.reset_selection();
 	_registry.to_front(&_menu);
 	_status_window.label(_menu.label());
+	_ncurses.clear_ok();
 	_menu.flush_all();
 }
 

gems/src/server/terminal_mux/ncurses.cc

@@ -66,6 +66,12 @@ Ncurses::Window *Ncurses::create_window(int x, int y, int w, int h)
 }
 
 
+void Ncurses::clear_ok()
+{
+	clearok(stdscr, true);
+}
+
+
 void Ncurses::do_update()
 {
 	doupdate();

gems/src/server/terminal_mux/ncurses_cxx.h

@@ -45,6 +45,8 @@ class Ncurses
 
 		Window *create_window(int x, int y, int w, int h);
 
+		void clear_ok();
+
 		void do_update();
 
 		Ncurses();