Minor fixes in trust anchor code

Follow up on !1762 (merged)

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.