ionice support (initially for rdiff-backup)
rdiff-backup can cause a lot of IO when scanning though folders. Running with ionice would be useful on top of the nice functionality (I see at least one other has requested it via debian bugs also). Here's a start - if happy I can continue and add it for other handlers that could benefit.
This patch also moves ionice commandline to $execstr so the debug output can show the complete commandline including ionice/nice config - which I think is useful for debugging. it is also only used now if ionice level is set, rather than always with a default of 0
ionice config is done with a "ionice_params" variable so users can manually set the commandline. I was testing with "-c3" for my set up.
(from redmine: created on 2014-03-02)