Two steps: First, expand tabs to 8 spaces in code and structure
listings.  Second, make the .Ds used to wrap code listings switch to
constant-width font (CW) rather than numeric font position 1, which
maps to R on most systems.

It is possible some archaic systems won't know what CW is, but the
only risk is that code listings won't look quite right on troff
devices; the PostScript and DVI drivers definitely grok it, so those
important cases are OK.

The purpose of these changes is to get rid of presentation-level
markup so these pages will lift clean to DocBook.

Signed-off-by: Eric S. Raymond <esr@thyrsus.com>