dpkg-divert improvements

Please use dpkg-divert anywhere it’s available, not just on Debian. Examples: • *buntu and other Debian derivatives • Atari FreeMiNT, for which a dpkg-based distribution (otherwise independent of Debian) exists • people using the dpkg toolchain on Interix, Maemo, …

Also, consider falling back to renaming if unwrapping with dpkg-divert fails (or perhaps check with dpkg-divert --list?); something else might have removed the diversion, or (on the non-Debian-derivative use case) someone might have removed the dpkg toolchain.

Finally, consider an option to not use dpkg-divert even if it exists, e.g. because the file is already diverted. (This effects the unwrapping point in the preceding paragraph.)