input/csv: stricter input data test for multi column mode

input/csv: stricter input data test for multi column mode

The previous implementation assumed that in multi-column mode each cell
communicates exactly one bit of input (a logic channel). But only the
first character got tested. Tighten the check, to cover the whole input
text. This rejects fully invalid input, as well as increases robustness
since multi-bit input like "100" was mistaken as a value of 1 before.