MINOR: log: add a new flag 'L' for locally processed requests

MINOR: log: add a new flag 'L' for locally processed requests

People who use "option dontlog-normal" are bothered with redirects and
stats being logged and reported as errors in the logs ("PR" = proxy
blocked the request).

This patch introduces a new flag 'L' for when a request is locally
processed, that is not considered as an error by the log filters. That
way we know a request was intercepted and processed by haproxy without
logging the line when "option dontlog-normal" is in effect.