This is an initial round of SAFERHANGUP hangup changes. It introduces
SAFERHANGUP, provides the core framework, and enables it for UNIX.
Window-port changes are provided for win/tty, win/X11 and win/gnome. Qt
changes should be forthcoming after having Warwick look at them.
window.doc is updated so windowport maintainers have an clue what needs to
be done to support SAFERHANGUP.