mount: return ENOSYS rather than EIO on attempted link
mount: return ENOSYS rather than EIO on attempted link

This fixes FileZilla accessing an rclone mount served over sftp.

See: https://forum.rclone.org/t/moving-files-on-rclone-mount-with-filezilla/5029

ncw/rclone,yonjah/rclone,yonjah/rclone,seuffert/rclone,B4dM4n/rclone,seuffert/rclone,ncw/rclone,olihey/rclone,B4dM4n/rclone,ncw/rclone,olihey/rclone,B4dM4n/rclone,yonjah/rclone,olihey/rclone,seuffert/rclone,B4dM4n/rclone,yonjah/rclone,seuffert/rclone,ncw/rclone,olihey/rclone