support exact match for rr_list() in RPC

Currently, `rr_list()` only supports (wildcard) patterns.
It can't distinguish between the user asking for a literal wildcard DNS record (`*.example.com.`) and a pattern `*.example.com.` (all records that match the pattern, e.g. `a.example.com.` and `a.example.com.`.

We should probably add a flag to do exact matching.