Handle UI's terminal closing
    
    Without this, GDB exits if a secondary UIs terminal/input stream is
    closed:
    
     $ ./gdb -ex "new-ui mi /dev/pts/6"
     New UI allocated
    	 <<< close /dev/pts/6
     (gdb) Error detected on fd 9
     $
    
    We want that for the main UI, but not secondary UIs.
    
    gdb/ChangeLog:
    2016-06-21  Pedro Alves  <palves@redhat.com>
    
    	* event-top.c (stdin_event_handler): Don't quit gdb if it was a
    	secondary UI's input stream that closed.  Instead, just delete the
    	UI.