gui: Replace global variable curses_active by a function.

gui: Replace global variable curses_active by a function.

There is no need to store this information as we can easily compute it.

shutdown_curses() needed to be changed to cope with early failures:
On fatal errors that happen after initscr() was called but before
the top window exists, we must shut down the curses subsystem albeit
curses_active() returns false in this case.