Revert "Improve error handling in p0f plugin"

Adding the p0f keyword to every log message is why I'm reverting this PR. [QP already prefixes log messages with the plugin name](https://github.com/smtpd/qpsmtpd/blob/master/lib/Qpsmtpd.pm#L115) that emitted them, making this redundant. There are also at least two QP log processors that use the /$pluginname: (pass|skip|fail), .*/ pattern to post-process the logs. This hurts those.