input/csv: rework user accessible options for consistency

input/csv: rework user accessible options for consistency

Rename the CSV input module's option keywords. To better reflect their
purpose, and for consistency across the rather complex set of options
and how they interact. Rearrange the list of options (not that the order
matters from the outside, but it's good to have during maintenance).

Update builtin help texts which will show up in applications, as well as
the source code comments which discuss these options in greater detail.
Would be nice to have a "general" help text for input modules which is
not tied to one single option, to provide an overview or use examples.
Arrange the option keys, short and long help texts such that the source
better reflects the applications' screen layout. To better support
future maintenance, again.

Consistently separate multi-work keywords for improved readability.
Prefer underscores over dashes for consistency with common keys in
shared infrastructure in other project sources (device options, MQ
items, etc).