In HTML output, correctly render .Bd -unfilled in proportionally-spaced

In HTML output, correctly render .Bd -unfilled in proportionally-spaced
font, rather than with the monospace font appropriate for .Bd -literal.
This fixes a minibug reported by anton@.

Implemented by no longer relying on the typical browser default of
"pre { font-family: monospace }" but instead letting <pre> elements
inherit the font family from their parent, then adding an explicit CSS .Li
class only for those displays where the manual page author requested it
by using the -literal option on the .Bd macro.