fw4: fix formatting of default log prefix

fw4: fix formatting of default log prefix

When using the explicit or implicit rule name as default log prefix, ensure
that is followed by a colon and a space to yield properly formatted firewall
log messages.

Also align the processing logic of `option log` in `config nat` sections with
that in `config rule` and `config redirect`.

Ref: https://forum.openwrt.org/t/x/137182/8
Signed-off-by: Jo-Philipp Wich <jo@mein.io>