#176 Stop install/update if low on disk space

[Walter Ian Kaye]

I had an update-all running, and suddenly found that my
available disk space was down to 8K (!).
Needless to say, that's a bad situation.

How about an option like "quit install if available
disk space is less than nnn K"?
(500 KB is my safety margin; I'm amazed I was even able
to close things down at 8K.)