[UBUNTU] Implement on-the-fly rewriting of maintainer fields

[UBUNTU] Implement on-the-fly rewriting of maintainer fields

Implement the same rules as pkgbinarymangler for rewriting
maintainer fields. This will fix the maintainer field of
source packages that were taken without changes from Debian
and contain therefor still the original maintainer information.

This should significantly reduce the number of Debian maintainers
that are listed on packages.ubuntu.com as Maintainers.

Thanks again to Colin Watson <cjwatson -at- ubuntu com> for the
suggestion.