posfconf debugger_command does not work. This is a 'known' problem with 2.6 and fixed in postfix 2.9. This is tied into the large code rewrite in 2.9, and thus cannot be patched in 2.6. You can see my thread on this over on the postfix list. There was a comment how those running 4 year old code can look forward to the new troubleshooting and support features in 2.10...