rs value will be the rdata type the SIG/RRSIG
covers.  The library treats the SIG and RRSIG types as if they
were a family of types, e.g. RRSIG(A), RRSIG(NS), RRSIG(SOA).
This makes RRSIGs much easier to work with than if RRSIGs
covering different rdata types were aggregated into a single
RRSIG rdataset.

*create*, a ``bool``.  If true, the node will be created if it does
not exist.

Raises ``KeyError`` if the name is not known and create was
not specified, or if the name was not a subdomain of the origin.

Returns a ``dns.rdataset.Rdataset``.
)r?