Since extract_name() does not convert to lowercase, do it temporarly within convert_domain_to_wire().

Introduce utility functions and RDATA meta-description.

Since owner and signer are both domain names and share the same
buffer in memory (daemon->namebuff), we need to go through a little
hoop to make sure one doesn't step on the other's toes. We don't
really need to extract the signer name until we have finished
calculating the hash of the RRset, so we postpone its extraction.

When the maxlen was exactly equal to the length of the string,
the function was returning 0 because the end-of-buffer check was
misplaced.