Fix line breaking in no-fill mode (.Bd -unfilled/<pre>),

Fix line breaking in no-fill mode (.Bd -unfilled/<pre>),
which apparently didn't work since the .Pp/<p> reorg.
The new logic is more similar to what the terminal formatter does:

1. Before a node that starts a new mdoc(7) input line,
start a new HTML output line.
2. An empty input line or a .Pp causes an empty output line.
3. Nothing needs to be done at the end of a node.

Severe misformatting was reported in table(5) by
Edgar Pettijohn <edgar at pettijohn dash web dot com> on misc@.