…upported

Before this change, if rclone was run with `-M` on a filesystem
without xattr support, it would error out.

This patch makes rclone detect the not supported errors and disable
xattrs from then on. It prints one ERROR level message about this.

See: https://forum.rclone.org/t/metadata-update-local-s3/32277/7

Before this fix, the chunksize calculator was using the previous size
of the object, not the new size of the object to calculate the chunk
sizes.

This meant that uploading a replacement object which needed a new
chunk size would fail, using too many parts.

This fix fixes the calculator to take the size explicitly.

…write

Before this change we assumed that github.com/Unknwon/goconfig was
threadsafe as documented.

However it turns out it is not threadsafe and looking at the code it
appears that making it threadsafe might be quite hard.

So this change increases the lock coverage in configfile to cover the
goconfig uses also.

Fixes #6378