how to handle userspace string copy failures

[Martin Hunt]

With the recent change in page fault handling, we are seeing more
failures from user_string(). Unfortunately, that results in an error
being logged and the script terminating. This is partly my fault because
I fixed this once before and then changed it back when asked to,
forgetting why this was bad. So, for the record, we cannot guarantee
always being able to always access userspace and such failures should
not terminate the script. At worst, I think we should print warnings. I
also propose that any user_string() request that fails should return
"<unknown>".

If there are no objections, I will check in this tapset change.

Martin