BUG/MEDIUM: local log format regression.

BUG/MEDIUM: local log format regression.

Since 2.3 default local log format always adds hostame field.
This behavior change was due to log/sink re-work, because according
to rfc3164 the hostname field is mandatory.

This patch re-introduce a legacy "local" format which is analog
to rfc3164 but with hostname stripped. This is the new
default if logs are generated by haproxy.

To stay compliant with previous configurations, the option
"log-send-hostname" acts as if the default format is switched
to rfc3164.

This patch addresses the github issue #963

This patch should be backported in branches >= 2.3.