Fixes since libtorrent 2.0:
- fix attribute in single-file v2 torrent creation
- fix padding for empty files in v2 torrent creation
- add function to ask a file_storage whether it's v2 or not
- fix mtime field when creating single-file v2 torrents
- fix performance regression in checking files
- disable use of SetFileValidData() by default (windows). A new setting
allows enabling it