Fix honoring system.file.allocate.set=1 rtorrent config setting:

• allocate space for those files only that don't have priority set to off.

It's useful with the following watch directory entry that only loads (in closed state) but not starts torrents, so priority of files can be set.
schedule2 = watch_dir_5, 7, 10, "load.normal=/path/to/watch-dir/load_but_not_start/*.torrent"

Refers to: rakshasa/libtorrent#109