input/csv: unobfuscate text line to column splitting

input/csv: unobfuscate text line to column splitting

The parse_line() routine is rather complex, optionally accepts an upper
limit for the number of columns, but unconditionally assumes a first one
and drops preceeding fields. The rather generic n and k identifiers are
not helpful.

Use the 'seen' and 'taken' names instead which better reflect what's
actually happening. Remove empty lines which used to tear apart groups
of instructions which are strictly related. This organization neither
was helpful during maintenance.