TUI: check whether in secondary prompt instead of immediate_quit

TUI: check whether in secondary prompt instead of immediate_quit

As can be seen in the tui_redisplay_readline comment:

 "The command could call prompt_for_continue and we must not restore
 SingleKey so that the prompt and normal keymap are used."

immediate_quit is being used as proxy for "secondary prompt".

We have a better predicate nowadays, so use it.

gdb/ChangeLog:
2016-04-12  Pedro Alves  <palves@redhat.com>

	* tui/tui-io.c (tui_redisplay_readline): Check
	gdb_in_secondary_prompt_p instead of immediate_quit.
	* tui/tui.c: Include top.h.
	(tui_rl_startup_hook): Check gdb_in_secondary_prompt_p instead of
	immediate_quit.