remove sensitive data from debug output more aggressively

This patch adds a new function that is used to remove sensitive data from
the debug output in a broader way. It will remove some innocent data but
should make sure most passwords and similar data can not be accessed even
when stored in some plugin's configuration data.

Disabling the debug option is still highly recommended.

darcs-hash:20080715211616-7ad00-19334e56d3910bcaa04147c4c59e0c59571764f3.gz