If there is a system dataset, use it.

Ticket: #5124

The issue here is that if there's a system dataset that is being used for syslog
the logfiles could grow to be quite large.  When ixdiagnose would try to
make it's tempfiles it would make them on /var, which is a memory
filesystem.

This would cause ixdiagnose to fail.
Improvements suggested by jkh
save debug button should use the system dataset if it exists.

Ticket: #5124