Prevent "sticky" fonts changes. (closes #956070; thanks to Sam

Prevent "sticky" fonts changes. (closes #956070; thanks to Sam
Steingold for reporting the problem, and for his patience...)

::Problem:
  Sometimes a bold or italic font change inadvertently ends up
  becoming "sticky" such that a following chunk of text that
  should just be rendered as plain text instead gets
  boldfaced/italicized.

::Cause:
  Font changes were simply being nested, as they are in HTML.
  While that works for HTML, it doesn't work for roff, where
  font-change instructions aren't actually intended to nest.

::Fix:
  Attempted to un-nest bold/italic font changes. When the manpages
  stylesheets encounter node sets that need to be boldfaced or
  italicized, they now put the \fBfoo\fR \fIbar\fR groff
  bold/italic instructions separately around each node in the set.

  This may not be a complete fix for the problem. In fact, it may
  cause other problems. Please test :^)