needspace: bad interactions with \addpenalty

[wspr]

Via Donald A on c.t.t.:

There is a conflict between \needspace and the \addpenalty
hidden inside the \section command.  \needspace should end
with \vskip\z@skip (\vskip 0 pt), just as the definition of
\vspace does, to ensure co-operation.

The first \needspace works because page breaks are forbidden
after chapter headings, so \addpenalty is not used by \section.

[wspr]

Closed in 1df629a