try to not require all par volumes

[jaredjennings]

Right now darbrrb tries to compromise between requiring a lot of disc switching and taking too long by reading too much data. But, last I checked, par 1 recoveries are cheap, so darbrrb could try the recovery before it has all the files off all the discs for a given par volume set. The net result would be that, when restoring, if nothing has gone wrong on the media, for a set with m data discs and n total discs, the user would only have to insert m different discs.

[jaredjennings]

(I should say this is an optimization. While supporting multiple drives, automounting, and the like are more interesting, after those are in place, this capability could let the user do fast, less-attended restores without buying enough drives for an entire set.)