CLEANUP: dns: use a single maximum DNS message size

The DNS code defines separate maximum message sizes for datagram and
stream processing, but both are the DNS protocol limit of 65535 bytes.
The duplicate names make common response buffers look
transport-specific and could allow the values to diverge.

The request ring size also has a TCP-specific name even though both
datagram and stream servers use it.

Replace the message limits with DNS_MAX_MSG_SIZE and rename the ring
limit to DNS_MSG_RING_MAX_SIZE. This does not change any limit or
runtime behavior.