(tui_initialize_readline): Allow to use space to leave SingleKey
to enter one gdb command.
(tui_enable): Restore the TUI keymap when we are back to TUI.
(tui_disable): Restore normal keymap when leaving TUI.
* tuiIO.c (tui_redisplay_readline): Restore the SingleKey mode
when the buffer becomes empty and we are in tui_one_command_mode.