Append redundant data to cdrom image to recover data from bad sectors.
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This software has a similar goal to Parchive
<https://en.wikipedia.org/wiki/Parchive>.  To add redundant parity data
to a set of files for the purpose of recovering those files from storage
media that has corrupted or missing pieces.  

Unlike Parchive, cdrparity appends parity data to the end of a cd-rom (or
dvd-rom or bd-rom) image without modifying the content of the image or
interfering with a reader that is not aware of this added data.  The 
additional parity data is essentially invisible to a reader that is not
aware of its presence.

The goal of this project is to fill all the remaining space on a disc with
redundant parity data which can later be used to recover the data from a
similarily sized section of bad sectors.  

The final layout of the disc becomes:

  + standard filesystem image (iso9660 or utf, encrypted or not)
  + marker
  + parity data
  + marker (identical to first)

The parity data is calculated as a simple xor of the main data chopped into
sections each with size equal to the size of the parity data.  This is similar
to how RAID 5 operates.

The program cdrparity adds the additional data to a pre-generated image.
Note that simultaneous generation of the image and burning is not supported.

The program cdrverify can be used to verify that the final image is correctly
formed.  This program was intentionally written in C (instead of C++) and
shares no code with cdrparity so as to serve as a verification of the correct
operation of cdrparity.

The program cdrrepair (v2 only) will verify the checksum on each marker, 
stripe, and the parity data to determine if any are corrupt.  Then, provided
the number of errors are few, the errors are corrected using the redundant
data.  Note that this program does not tolerate I/O errors.

The program cdrrescue can be used to recover a disc that has bad sectors.
What it does is attempt to read sectors (both the data and parity) until
enough data is read to recover the original image.  Where bad sectors are
found, the program simply moves on instead of terminating with an error
or getting stuck in an infinite loop of retries.  Note that this program
has not yet been updated to support v2.  


Version 2
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With v2 a checksum (siphash24) of each stripe and the parity are calculated
and stored in the marker.  Without this checksum, the location of errors could
only be determined if an I/O error occurs.  With the checksum, stripes can be
identified as corrupt even in cases where the read was successful.  This
increases the probability of successfully recovering the image.