btrfs-progs: mkfs: switch to single as default profile for multiple-devices
    
    The single profile is better suited as default for data on multiple
    devices. Switch from RAID0 because:
    
    - it's easier to convert to other profiles, as single consumes some
      chunks per device, but RAID0 has chunks on all devices regardless of
      the used space
    
    - RAID0 has no redundancy and compared one disk failure affects many
      files due to striping, while with single the chances are a bit higher
      that complete files are stored on one device
    
    - when the device sizes are not equal and not even close to equal, the
      maximum achievable size with RAID0 is size of the smallest device due
      to striping, with single it's the sum of all device sizes
    
    The changed defaults could affect scripts and deployments that rely on
    the old values, but given the number of possible profiles for multiple
    devices let's hope that they're specified explicitly in majority of
    cases.
    
    Issue: #270
    Signed-off-by: David Sterba <dsterba@suse.com>