MINOR: dns: select the nameserver server type explicitly

DNS send, receive and response functions currently infer which server
to use from the non-null member of a nameserver. This is unambiguous
while a nameserver owns only one server, but prevents one logical
nameserver from exposing both datagram and stream servers.

Pass a dns_server_type through these interfaces and add predicates for
readable type checks. Existing queries still select the same server, so
this patch does not change runtime behavior.