Debian: Update for Bullseye

Besides the obvious s/buster/bullseye/ sort of changes, this:
  - Removes the use of backports.  At this time, the difference is
    minimal (2.0.6 vs 2.0.3) and I do not see a need to force backports
    on users.  They are free to add it later if they wish.
  - Updates the sources.list for bullseye-security also being at
    deb.debian.org.
  - Updates the bpool features:
    - device_rebuild has an explanation of why it is not used.
    - livelist is now used.
    - log_spacemap has been tested, but shares the same explanation as
      spacemap_v2 for why it is not used.

Fixes #204
Closes #126
Closes #205

Thanks: Juri Grabowski <[email protected]>
Thanks: Freezed <[email protected]>
Signed-off-by: Richard Laager <[email protected]>