lib/utils: fix timestamp format in dumps of records
The debug dumps of packets used UNIX timestamps (in RRSIG validity) instead of the customary human stamps. This was an unintentional regression of 0555828e, i.e. since v5.4.1
I looked again at all other differences from default kdig style, and the only ones are that we don't show class and don't do IDN. (both seem suitable here)