This feature mimics the standard mhddfs behavior but is more thorough.
If a write fails and the errno is set to ENOSPC then mergerfs will (if
the feature is enabled) attempt to move the file to the drive with
the most free space but only if it has enough room for the file plus
the amount to be written. If that transfer is successful it will then
unlink the original file and attempt the previously failed write again.
The copy includes copying the path and file including the acls, owners,
attributes, extended attributes, and timestamps.
Return EXDEV if directories of tragets differ. If the old and new path exist in the same directory first rename each old found by the policy and then unlink/rmdir any new on a drive which didn't rename. The unlink/rmdir will occur only if there were no rename errors. Any failures of unlink/rmdir are ignored. The last rename error is returned.
