BUG/MINOR: log: LF upsets maxlen for UDP targets

BUG/MINOR: log: LF upsets maxlen for UDP targets

A regression was introduced with 5464885 ("MEDIUM: log/sink: re-work
and merge of build message API.").

For UDP targets, a final '\n' is systematically inserted, but with the
rework of the build message API, it is inserted after the maxlen
limitation has been enforced, so this can lead to the final message
becoming maxlen+1. For strict syslog servers that only accept up to
maxlen characters, this could be a problem.

To fix the regression, we take the final '\n' into account prior to
building the message, like it was done before the rework of the API.

This should be backported up to 2.4.