Handle UI's terminal closing

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.