Minor fixes in trust anchor code

Minor fixes in trust anchor code

This commit makes some minor changes to the trust anchor code:

1. Replace the undescriptive n1, n2 and n3 identifiers with slightly
   better rdata1, rdata2, and rdata3.
2. Fix an occurrence where in the error log message a static number
   32 was printed, rather than the rdata3 length.
3. Add a default case to the switch statement checking DS digest
   algorithms to catch unknown algorithms.