input/csv: fixup input file line number handling

input/csv: fixup input file line number handling

The previous implementation allowed CSV input files to use any line
termination in either CR only, LF only, or CR/LF format. The first EOL
was searched for and was recorded, but then was not used. Instead any of
CR or LF were considered a line termination. "Raw data processing" still
was correct, but line numbers in diagnostics were way off, and optional
features like skipping first N lines were not effective. Fix that.

Source code inspection suggests the "startline" feature did not work at
all. The user provided number was used in the initial optional search
for the header line (to get signal names) or auto-determination of the
number of columns. But then was not used when the file actually got
processed.