Fix line count in slashUsage().

Fix line count in slashUsage().

Counting newlines shows that quite a few recent patches have neglected
to update the output-lines count given to PageOutput().  Fortunately
it's not terribly critical that this be exact, since we long since
exceeded the height of most people's terminal windows.  Still, maybe
we ought to think of a way to not have to maintain this manually anymore.