Remove cleanups from TUI

Remove cleanups from TUI

This removes the last cleanups from the TUI, by using std::string
rather than manual memory management.

Regression tested against gdb.tui/*.exp on Fedora 26 x86-64.

gdb/ChangeLog
2017-10-09  Tom Tromey  <tom@tromey.com>

	* tui/tui-win.c (tui_set_win_height, parse_scrolling_args): Use
	std::string.
	* tui/tui-layout.c (enum tui_status): Use std::string.