nl: only fail if need to output overflowed numbers

nl: only fail if need to output overflowed numbers

Previously we would have failed immediately upon internal overflow,
which didn't output the full line being processed, and assumed
there would be another numbered line.

* src/nl.c (line_no_overflow): A new global to track overflow.
(print_lineno): Only fail if about to output an overflowed number.
(reset_lineno): A new function to refactor resetting of the number,
and which also clears line_no_overflow.
* tests/misc/nl.sh: Add a test case.