Fix terminal state corruption when starting a program from within TUI

The TUI terminal state becomes corrupted (e.g. key sequences such as
Alt_F and Alt_B no longer work) when one attaches to an inferior process
(via "run" or "attach") from within TUI.  This terminal corruption
remains until you switch out of TUI mode.

This happens because the terminal state is not properly saved when
switching to and out from TUI mode.  Although the functions tui_enable()
and tui_disable() both call the function target_terminal_save_ours() to
save the terminal state, this function is a no-op unless GDB has already
attached to an inferior process.  This is because only the "native"
target has a useful implementation of target_terminal_save_ours()
(namely child_terminal_save_ours()) and we only have the "native" target
in our target vector if GDB has already attached to an inferior process.

So without an inferior process, switching to and from TUI mode does not
actually save the terminal state.  Therefore when you attach to an
inferior process from within TUI mode, the proper terminal state is not
restored (after swapping from the inferior's terminal back to the GDB
terminal).

To fix this we just have to ensure that the terminal state is always
being properly saved when switching from and to TUI mode.  To achieve
this, this patch removes the polymorphic function
target_terminal_save_ours() and replaces it with a regular function
gdb_save_tty_state() that always saves the terminal state.

Tested on x86_64-unknown-linux-gnu by running "make check", no new
regressions.

gdb/ChangeLog:
	* target.h (struct target_ops::to_terminal_save_ours): Remove
	declaration.
	(target_terminal_save_ours): Remove macro.
	* target-delegates.c: Regenerate.
	* inf-child.c (inf_child_target): Don't set the nonexistent
	field to_terminal_save_ours.
	* inferior.h (child_terminal_save_ours): Remove declaration.
	* terminal.h (gdb_save_tty_state): New declaration.
	* inflow.c (child_terminal_save_ours): Rename to ...
	(gdb_save_tty_state): ... this.
	* tui/tui.c: Include terminal.h.
	(tui_enable): Use gdb_save_tty_state instead of
	target_terminal_save_ours.
	(tui_disable): Likewise.

gdb/inflow.c

@@ -241,7 +241,7 @@ child_terminal_init_with_pgrp (int pgrp)
    and gdb must be able to restore it correctly.  */
 
 void
-child_terminal_save_ours (struct target_ops *self)
+gdb_save_tty_state (void)
 {
   if (gdb_has_a_terminal ())
     {