After years of gnashing of teeth, i finally found a way to avoid

After years of gnashing of teeth, i finally found a way to avoid
having to write empty list elements for non-compact .Bl -tag lists:

1. Add margin-bottom to the <dd>.
Note that margin-top on the <dt> doesn't work because it would put
a short <dt> lower than the <dd>; margin-bottom on the <dt> doesn't
work because it would put vertical space before the <dd> for a long
<dt>; and margin-top on the <dd> doesn't work because it would put
a short <dt> higher than the <dd>.  Only margin-bottom on the <dd>
has none of these adverse effects.

2. Of course, margin-bottom on the <dd> fails to take care of the
vertical spacing before the first list element, so implement that
separately by margin-top on the <dl>.

3. For .Bl -tag -compact, reset both to zero.