input/csv: introduce generic "column processing" support

Rephrase the CSV input module's implementation such that generic support
to "process a column" becomes available. All columns of an input file's
text line get inspected, a column can either get ignored, or converted
to logic data. A future version can then remove the current limitations
of single- and multi-column modes (either one single multi-bit cell, or
multiple single-bit cells which must be adjacent).

Combine the bin/oct/hex parse routines into one routine which handles up
to four bits per input number digit with common logic. Availability of
more data than channels (according to user specs) is not fatal.

Drop the counter intuitive "first-channel" option, use "first-column"
instead. Warn when comment leader and column separator are identical
(was silent before, may be unexpected). Extend diagnostics and address
minor readability nits, update comments. Rephrase logic channel name
assignment.

Use simple scalar options to derive generic processing details: Either
'single-column' and 'numchannels' are required, with an optional
'format' spec (resulting in single-column mode). Or 'first-column' with
an optional 'numchannels' (multi-column mode with fixed format, using
all available columns by default). The default is multi-column mode with
one logic channel per column and spanning all columns on a text line.