Wrap <filename> in \url{...} if justification is turned on. This
prevents the majority of the remaining margin overflows. However,
this solution isn't very elegant in that it completely ignores
<replaceable>s inside of a <filename> element. The normal TeX that
gets output inside of \url{..} confuses TeX, so we must simply process
the children of replaceable without doing any additional formatting
inside of the \url{..} (such as putting the text into italics).

More work needs to be done on this.