DOCBOOK-APPS: Bug fix for print stylesheet: <anchor>

I have a bug fix submission to include in the next version of the DocBook
stylesheets.

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The problem: links using the RTF backend would always go to the top of the
document because an anchor was not being inserted into the document.  I
found the solution to this problem in dbindex.dsl:

(element indexterm
  ;; This is different than (empty-sosofo) alone because the backend
  ;; will hang an anchor off the empty sequence.  This allows the index
  ;; to point to the indexterm (but only if the indexterm has an ID).
  (make sequence (empty-sosofo)))

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I recommend that the same treatment be given to element anchor in
dblink.dsl:

(element anchor
  ;; This is different than (empty-sosofo) alone because the backend
  ;; will hang an anchor off the empty sequence.  This allows links
  ;; to point to the anchor (but only if the indexterm has an ID).
  (make sequence (empty-sosofo)))

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Note that the current rule for anchor looks like this:

(element anchor (empty-sosofo))

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Cheers!

-karl