bug530687-srivasta: Support for architecture wildcards

Support for architecture wildcards has been added to dpkg-1.13.13. This
patch, based on a proposal from Andres Mejia, provides policy on how
architecture wildcards should be used for other tools such as sbuild and
pbuilder. This patch has tracked and incorporated suggestions embedded
the discussion of the proposal. It also brings policy up to speed and in
line with dpkg-dev which appears to generate an Architecture line that
includes both architectures and special values like "all".

See: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=530687
     http://lists.debian.org/debian-policy/2009/05/msg00108.html
for details.

Signed-off-by: Andres Mejia <mcitadel@gmail.com>
Signed-off-by: Manoj Srivastava <srivasta@debian.org>