input/csv: data type nits (sizes, enums)

input/csv: data type nits (sizes, enums)

Use size_t for things that get counted: column indices, channel numbers
(line numbers already used size_t). De-anonymize an enum to avoid 'int'
where it gets referenced. Adjust printf(3) format strings. Get unsigned
values from option lookups (stick with 32bits, should be acceptable for
spreadsheet columns and channel counts).

Address other minor nits while we are here: Also terminate the last item
in an enum declaration. Add a doxygen comment for parse_line(). Rename a
parameter to achieve tabular doc text layout.