CRLICU ( posn, direct, line, ifail )
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  Create a linear curve (i.e. straight line).

Receives:
  KI_vec_position           posn           --- position on line
  KI_vec_direction          direct         --- line direction

Returns:
  KI_tag_curve             *line           --- linear curve
  KI_cod_error             *ifail          --- failure code


Description:
  An unbounded straight line is created which passes through the given
  position in the given direction.