Convert indentation tabs within <programlisting> elements to 4 spaces.

Convert indentation tabs within <programlisting> elements to 4 spaces.

As the book says:
 "Often a program listing will include tab characters to indent lines of code.
  When such a listing is imported to DocBook XML and formatted, the tab
  characters are not expanded as they are in the program editor.
  That's because in XSL-FO and HTML, tab stops and tab expansion are not
  described in either HTML or XSL-FO standards.
  By default, an XSL-FO processor treats a tab character as a single space,
  which leads to unsatisfactory results."

http://www.sagehill.net/docbookxsl/TabExpansion.html

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