logs-show: adjust source timestamp with header timestamp

logs-show: adjust source timestamp with header timestamp

Previously, _SOURCE_REALTIME_TIMESTAMP was only used for realtime
timestamp, and _SOURCE_MONOTONIC_TIMESTAMP was for monotonic.

This make these journal field used more aggressively. If we need
realtime timestamp, but an entry has only _SOURCE_MONOTONIC_TIMESTAMP,
then now realtime timestamp is calculated based on
_SOURCE_MONOTONIC_TIMESTAMP and the header dual timestamp.
Similary, monotonic timestamp is obtained from
_SOURCE_REALTIME_TIMESTAMP and the header dual timestamp.

This should change shown timestamps not so much in most cases, but may
be improve the situation such as #32492.