time-util: introduce deserialize_timestamp_value()

The time-util.c provides dual_timestamp_deserialize() function to
convert value to usec_t and set it as value of ts->monotonic and
ts->realtime.

There are some places in code which do the same but only for one
clockid_t (realtime or monotonic), when dual_timestamp_deserialize()
sets value of both.

This patch introduces the deserialize_timestamp_value() which converts
 a given value to usec_t and write it to a given timestamp.

src/basic/time-util.h

@@ -99,6 +99,7 @@ char *format_timespan(char *buf, size_t l, usec_t t, usec_t accuracy);
 
 void dual_timestamp_serialize(FILE *f, const char *name, dual_timestamp *t);
 int dual_timestamp_deserialize(const char *value, dual_timestamp *t);
+int deserialize_timestamp_value(const char *value, usec_t *timestamp);
 
 int parse_timestamp(const char *t, usec_t *usec);