Revert previous, as requested by kristaps@.

Revert previous, as requested by kristaps@.
The .Bf block can contain subblocks, so it has to render as an
element that can contain flow content.  But <em> cannot contain
flow content, only phrasing content.  Rendering .Em and .Bf differently
would by unfortunate, and closing out .Bf before subblocks and
re-opening it afterwards would merely complicate both the C code
of the program and the generated HTML code.  Besides, converting
.Em to semantic HTML markup would require some content to be put
into <em> and some into <i>, but we cannot automatically distinguish
which is which, so strictly speaking, we can't use semantic HTML
here but have to fall back to physical markup.  Wonders of HTML...