Add parallel=n and move DEB_BUILD_OPTS section

Add parallel=n and move DEB_BUILD_OPTS section

Add the build option parallel=n, requesting a parallel build.  Standardize
the format of DEB_BUILD_OPTIONS, require tags be whitespace-separated,
allow packages to assume non-conflicting tags, and require unknown flags
be ignored.  Move the section documenting DEB_BUILD_OPTS under the
debian/rules section and away from the binaries section, leaving a
cross-reference behind.  Thanks to Loïc Minier, Peter Samuelson,
Robert Millan, and Guillem Jover for wording suggestions.

git-archimport-id: rra@debian.org--lenny/debian-policy--devel--3.7--patch-40