config: Add disk space check for any new item

[pyroscope]

See rakshasa/rtorrent#727 (comment)

• Use d.free_diskspace in a greater check against d.size_bytes
• Use math.sub if available to keep a reserved amount free
• stop the item in its tracks (+ log that) if disk space is low, and set it to prio=0 (to avoid further auto-starts by other config or cron)
• there is a problem regarding other items that still fit, get started, and in sum eat up all disk space – so possibly force sychronized disk allocation when disk free gets near the reserved space (does f.set_create_queued help?)
• test it with a load.start schedule, whether any space is claimed before the trigger fires

[chros73]

One of my pull req has the fix and command for d.disk_freespace.

[chros73]

That's the one d.is_enough_diskspace : rakshasa/rtorrent#604
The problem was: "fix free_diskspace() method in libtorrent to report back proper size even if a download is stopped"