cut: with -f, process each line independently

cut: with -f, process each line independently

Previously line N+1 was inspected before line N was fully output,
which causes output ordering issues at the terminal or delays
from intermittent sources like tail -f.

* src/cut.c (cut_fields): Adjust so that we record the
previous output character so we can use that info to
determine wether to output a '\n' or not.
* tests/misc/cut.pl: Add tests to ensure existing
functionality isn't broken.
* NEWS: Mention the fix.
Fixes bug http://bugs.gnu.org/13498