input/csv: introduce column-formats option (flexible logic import)

input/csv: introduce column-formats option (flexible logic import)

Extend the CSV input module, introduce support for the "column-formats="
option. This syntax can express the previous single- and multi-column
semantics, as well as any arbitrary order of to-get-ignored, and single-
and multi-bit columns in several formats.

The previous "simple" keywords for single and multi column modes still
are in place, it's yet to get determined whether to axe them. Depends on
whether users can handle the format strings for these simple cases.