Replace tui_restore_gdbout with scoped_restore

Replace tui_restore_gdbout with scoped_restore

This patch replaces tui_restore_gdbout (a cleaup function) with a use
of scoped_restore.  This one is broken out into its own patch because
it might slightly change the behavior of gdb: it saves and restores
pagination_enabled, whereas the tui_restore_gdbout unconditionally set
pagination_enabled to 1; and I think this warrants closer review.

ChangeLog
2017-08-03  Tom Tromey  <tom@tromey.com>

	* tui/tui-regs.c (tui_restore_gdbout): Remove.
	(tui_register_format): Use scoped_restore.