set MUNIN_VERSION correctly during build. (Closes: #694527)

debian/rules: set MUNIN_VERSION correctly during build. (Closes: #694527)
fixup debian/munin-async.logrotate (Closes: #691758)

* debian/munin-async.logrotate:
  - use adm group for created files.
  - use *0 as glob, not *.0 (Closes: #691758)
release as 2.0.6-3 to sid