The de-duplication of txt, nullrr, cert, key and sig records
reduced all records to a single one.
Also, dblookup1() missed the txt record case and did not return
a unique list of rr's.
Now we consider these records unique if their value is different.
The new txtequiv() function does that for TXT records, which is
a bit tricky as it needs to take different segmentation into account.
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