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.