Stop treating SIG and NXT records specially

Stop treating SIG and NXT records specially

RFC 3755 retired SIG and NXT in favour of RRSIG and NSEC.  BIND still
warned about them at zone load, refused them in dynamic updates,
parsed SIG with a non-zero "type covered" field as a signature on an
RRset, and tracked them via dns_rdatatype_issig().  Those carve-outs
were the sole path that made the GL#5818 crash class reachable.

Treat both types as ordinary unknown rdata: they load, transfer, sign
and answer like any other record, and dynamic updates carry them
through the generic path.  SIG(0) is unaffected; its message-parsing
carve-out is preserved.