[docbook-tc] modified <co> for HTML

I just checked into CVS a modified html/callout.xsl.
This includes a modification to the <co>
XSL template for HTML output to use any linkends
attribute to form a link from the <co> bug
to its matching <callout> element.  That forms the
two-way link as described in The Definitive Guide.
Currently it only works with a single-valued
'linkends', because HTML doesn't support multiply-targeted
links.  

This change only works for using <co> in your screen or
programlisting, not for using screenco or programlistingco
with areaspec, because those are currently implemented
using extension functions.  

-- 

Bob Stayton                                 400 Encinal Street
Publications Architect                      Santa Cruz, CA  95060
Technical Publications                      voice: (831) 427-7796
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