dm raid: cleanup / provide infrastructure

Provide necessary infrastructure to handle ctr flags and their names
and cleanup setting ti->error:

 - comment constructor flags

 - introduce constructor flag manipulation

 - introduce ti_error_*() functions to simplify
   setting the error message (use in other targets?)

 - introduce array to hold ctr flag <-> flag name mapping

 - introduce argument name by flag functions for that array

 - use those functions throughout the ctr call path

Signed-off-by: Heinz Mauelshagen <[email protected]>
Signed-off-by: Mike Snitzer <[email protected]>