input/csv: eliminate magic numbers in options declaration

input/csv: eliminate magic numbers in options declaration

The CSV input module has grown so many options, that counting them by
hand became tedious and error prone. Eliminate the magic numbers in the
associated code paths.

This also has the side effect that the set is easy to re-order just by
adjusting the enum, no other code is affected. Help text and default
values is much easier to verify and adjust with the symbolic references.

[ see 'git diff --word-diff' for the essence of the change ]