re: Jira NCF-309

The code for handling character constants
in datalists in ncgen has some problems.
1. It failed on large constants
2. It did not handle e.g. var = 'a', 'b', ...
   in the same way that ncgen3 did.
3. The code for generate.c and genchar.c needed
   some refactoring to make it a little simpler
   (but not simple).