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.

src/basic/time-util.h

@@ -72,6 +72,7 @@ typedef enum TimestampStyle {
 #define TIME_T_MAX (time_t)((UINTMAX_C(1) << ((sizeof(time_t) << 3) - 1)) - 1)
 
 #define DUAL_TIMESTAMP_NULL ((dual_timestamp) {})
+#define DUAL_TIMESTAMP_INFINITY ((dual_timestamp) { USEC_INFINITY, USEC_INFINITY })
 #define TRIPLE_TIMESTAMP_NULL ((triple_timestamp) {})
 
 usec_t now(clockid_t clock);