poetry.dtx (\maybe@unskip): Match the version in `mdwtab'.

I don't actually know why these were different.  Both versions appear to
have been there forever, though it's clear that these packages had lives
before they were checked into CVS in 2002.  Obviously, since these have
the same name, they should have the same definition.  The `mdwtab'
version is more robust (checking `\ifhmode' before applying `\hskip'),
and slightly more refined (using `\m@ne' rather than `-\@ne'), so I'm
going with that one.