input/csv: improve reliabilty of text line isolation

input/csv: improve reliabilty of text line isolation

Slightly unobfuscate the "end of current input chunk" marker in the data
processing loop. Make the variable's identifier reflect that it's not a
temporary, but instead something worth keeping around until needed again.

Unbreak the calculation of line numbers in those situations where input
chunks (including previously accumulated unprocessed data) happens to
start with a line termination. This covers input files which start with
empty lines, as well as environments with mutli-byte line termination
sequences (CR/LF) and arbitrary distribution of bytes across chunks.

This fixes bug #968.

Accept when there is no line termination in the current input chunk. We
cannot assume that calling applications always provide file content in
large enough chunks to span complete lines. And any arbitrary chunk size
which applications happen to use can get exceeded by input files (e.g.
for generated files with wide data or long comments).