curses on unix - terminal reset needed after abort

curses on unix - terminal reset needed after abort

Noticed while investigating the message loop.  If I had level files
from an interrupted game and was asked "Destroy old game?" when
starting a new one, answering 'n' left the terminal in an unusable
state.  Executing 'stty sane' (invisibly since input echo was off)
repaired things but the user shouldn't have to do that.

Change unixtty.c's error() to shut down windowing if that has been
initialized.  This might need some tweaking for tty, which will now
clear the screen before showing the startup error message.  Other
systems besides unix use unixtty.c so are affected, but I think the
change doesn't introduce anything that should cause trouble (aside
from the potential screen erasure).