MEDIUM: log: change default "host" strategy for log-forward section

MEDIUM: log: change default "host" strategy for log-forward section

Historically, log-forward proxy used to preserve host field from input
message as much as possible, and if syslog host wasn't provided
(rfc5424 '-' or bad rfc3164 or rfc5424 message) then "localhost" or "-"
would be used as host when outputting message using rfc3164 or rfc5424.

We change that behavior (which corresponds to "keep" host option), so that
log-forward now uses "fill" strategy as default: if the host is provided
in input message, it is preserved. However if it is missing and IP address
from sender is available, we use it.