[docbook-tc] DTD changes needed to implement Olink proposal

Wanted to get this out for comments before next week's TC call...

My one action item from the September telcon was to document changes
needed to the DocBook DTD to implement Bob Stayton's Olink proposal.

  http://www.sagehill.net/OlinkExtended.html

Based the proposal and a discussion with Bob, it seems like the only
potential DTD changes needed are:

 * a new Targetdoc CDATA attribute
 * a new Targetid IDREF attribute
 * a "stylesheet" value for the Type attribute

Bob says a processing application can infer the "stylesheet" value for
the Type attribute when it determines that the targetdoc and targetid
attributes are in use. So I think that'll leave us with a new ATTLIST
for Olink that looks like this:

  <!ATTLIST olink
                   targetdoc        CDATA           #IMPLIED
                   targetid         IDREF           #IMPLIED
                   targetdocent     ENTITY          #IMPLIED
                   linkmode         IDREF           #IMPLIED
                   localinfo        CDATA           #IMPLIED
                   type             CDATA           #IMPLIED
                   %common.attrib;
                   %olink.role.attrib;
                   %local.olink.attrib;
  >

It's also worth noting that Bob has already produced a documented
working implementation of the proposal, with examples.

  --Mike