Include head of rdata buffer in internal `rdata_t`

[k0ekk0ek]

The per-RR structure used to track current offset (tail) and limit, introduced to make it easier to define a custom window when parsing SVCB parameters as they may need to get reordered, should include the head of the buffer as well. This would allow for better verification (asserts or otherwise) of correct buffer management. Especially when we implement support for using multiple buffers so that the parsing and database inclusion can be done in parallel.