refresh 0.8.4

This patch set tracks the changes needed in the Debian packaging for ZFS 0.8.4, which is not yet has been released. (and currently buggy on at least Linux 5.6 according to my testing) Upstream is aware of this issue, it is not Debian-specific.

• Several patches are now included upstream, and are removed.
• Some files are installed differently. We continue installing them as we have been, but ignore their duplicates in debian/not-installed.

I have tested this on 5.5 and 5.6. It builds and installs. (but there are some ZTS failures, definitely for 5.6, and possibly for 5.5). These are being addressed upstream, in the aforementioned upstream PR. I am currently testing on more stable hardware with the ZTS infrastructure of !22 (merged). Careful re-testing does not suggest that there are issues on 5.5.