Low: libcrmcommon: always mark durations appropriately

Previously, a crm_time_t would get marked as a duration if it was created with
crm_time_calculate_duration() but not if was created if crm_time_parse_duration().
Now it is correctly marked in the latter case.

Also ensure crm_duration_as_string() prints "0 seconds" for a 0-length
duration.

This fixes the case where a 0-length duration was given to iso8601.