json: use packet timestamp if available

This patch updates the JSON output plugin to have it use the
timestamp of the packet if available. The date format used
for the timestamp is now using ISO 8601 to have an easy
import in most software (tested with logstash and splunk).

Signed-off-by: Eric Leblond <eric@regit.org>
printpkt: oob.time.sec was not used

This patch supresses the oob.time.sec from the input keys as this
is not used when creating the string corresponding to the packet.

Signed-off-by: Eric Leblond <eric@regit.org>
ulogd: fix loglevel handling

It was always default if not specified by command parameter.

Signed-off-by: Ken-ichirou MATSUZAWA <chamas@h4.dion.ne.jp>