Fixed handling of backspace for ncurses builds

[Lyle-Tafoya]

In ncurses, backspace key is returned from wgetch as KEY_BACKSPACE. This is one of the situations where PDCurses and ncurses have different behavior. PDCurses has KEY_BACKSPACE defined, but for whatever reason does not make use of it. It is simple enough to add a check for KEY_BACKSPACE in addition to the other values it checks. Without this, ncurses builds of the program do not handle the backspace character properly.