use <a> for hancors without content, not <span>

[Patrice Dumas]

Hello,

Currently <span id="id"></span> is used for anchors, whether they
originate from @anchor, in some cases from @node and sectionning
@-commands, @*index, ...  It is syntactically correct, but I think that
it would be semantically better to use <a> and keep <span> for inline
text that needs some kind of formatting information.

Also I propose to add class to the anchors to distinguish their source.

Any opposition/idea?

-- 
Pat