Improve handling of some .git/annex/ subdirectories being on other
filesystems, in the bittorrent special remote, and youtube-dl integration,
and git-annex addurl.
The only one of these that I've confirmed to be a problem is in the
bittorrent special remote when .git/annex/tmp and .git/annex/othertmp are
on different filesystems.
As well as auditing for renameFile, also audited for createLink, all of
those are ok as are the other remaining renameFile calls. Also audited all
code paths that use .git/annex/othertmp, and did not find any other
cross-device problems. So, removing mention of othertmp needing to be on
the same device.
Sponsored-by: Dartmouth College's Datalad project
