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

[chros73]

Depends on: rakshasa/libtorrent#152
Probably clashes with: #575 (it depends on it)

With the previous patch rtorrent could be easily crashed:

• e.g. when a magnet link was loaded, started and there wasn't enough disk space for it

Modify fixing honoring system.file.allocate.set=1 patch:

• do not allow to start a torrent if there's not enough disk space for it
  • notify the user about this
• modify priorities of files as well in libtorrent
• add 2 new file property in libtorrent:
  • is_fallocatable : if a file has flag_fallocate flag
  • is_fallocatable_file : if a file has flag_fallocate and flag_resize_queued flags
• modify DownloadList::create() to take into account system.file.allocate setting
• modify d.update_priorities to be able to pass along file flags
• refactor DownloadList::resume() a bit

New commands in rtorrent:

• d.is_enough_diskspace : boolean
• d.allocatable_size_bytes : size in bytes needed to create the download
• f.is_fallocatable , : boolean, returns true if a file has flag_fallocate flag
• f.is_fallocatable_file : boolean, returns true if a file both has flag_fallocate and flag_resize_queued flags
• f.set_fallocate, f.unset_fallocate : setters for flag_fallocate

Additional fix:

• fix free_diskspace() method in libtorrent to report back proper size even if a download is stopped

Note:
Unfortunately, this patch isn't complete yet, rtorrent can be still crashed:

• load a multi file torrent that is larger than free disk space but not start it
• deselect some files to be able to start download and start it
  • file allocation will be immediate
• now select some previously deselected files that won't fit on the disk
  • rtorrent will crash

[chros73]

@rakshasa , I'll try to do something about the 2nd (incomplete) part this week. If I'll won't be able to figure out a good solution then I'll leave this with you (it's getting more and more complicated :) ).
Thanks