new: Improve the fix with the NULL-sentinel.

new: Improve the fix with the NULL-sentinel.
With multiple sequence arguments on the command line you might end up
with garbage in the sequence[] array (it needs the last argument to be
a NULL. So do two things: make the first two arguments NULL, and as we
add arguments to sequence[] make sure the list has a NULL on the end.
This is another problem discovered by the nmh test suite.

Pulled in from nmh. Thanks to Ken Hornstein.