conf, udp: Drop mostly duplicated dns_send arrays, rename related fields

Given that we use just the first valid DNS resolver address
configured, or read from resolv.conf(5) on the host, to forward DNS
queries to, in case --dns-forward is used, we don't need to duplicate
dns[] to dns_send[]:

- rename dns_send[] back to dns[]: those are the resolvers we
  advertise to the guest/container

- for forwarding purposes, instead of dns[], use a single field (for
  each protocol version): dns_host

- and rename dns_fwd to dns_match, so that it's clear this is the
  address we are matching DNS queries against, to decide if they need
  to be forwarded

Suggested-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

ndp.c

@@ -121,7 +121,7 @@ int ndp(struct ctx *c, const struct icmp6hdr *ih, const struct in6_addr *saddr)
 		if (c->no_dhcp_dns)
 			goto dns_done;
 
-		for (n = 0; !IN6_IS_ADDR_UNSPECIFIED(&c->ip6.dns_send[n]); n++);
+		for (n = 0; !IN6_IS_ADDR_UNSPECIFIED(&c->ip6.dns[n]); n++);
 		if (n) {
 			*p++ = 25;				/* RDNSS */
 			*p++ = 1 + 2 * n;			/* length */
@@ -130,8 +130,8 @@ int ndp(struct ctx *c, const struct icmp6hdr *ih, const struct in6_addr *saddr)
 			p += 4;
 
 			for (i = 0; i < n; i++) {
-				memcpy(p, &c->ip6.dns_send[i], 16);
-				p += 16;			/* address */
+				memcpy(p, &c->ip6.dns[i], 16);	/* address */
+				p += 16;
 			}
 
 			for (n = 0; *c->dns_search[n].n; n++)