MEDIUM: dns: extract options

DNS selection preferences are actually declared inline in the
struct server. There are copied from the server struct to the
dns_resolution struct for each resolution.

Next patchs adds new preferences options, and it is not a good
way to copy all the configuration information before each dns
resolution.

This patch extract the configuration preference from the struct
server and declares a new dedicated struct. Only a pointer to this
new striuict will be copied before each dns resolution.
diff --git a/src/checks.c b/src/checks.c
index 2cfb01f..4d3b393 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -2218,8 +2218,8 @@
 	resolution->query_id = query_id;
 	resolution->qid.key = query_id;
 	resolution->step = RSLV_STEP_RUNNING;
-	resolution->resolver_family_priority = s->resolver_family_priority;
-	if (resolution->resolver_family_priority == AF_INET) {
+	resolution->opts = &s->dns_opts;
+	if (resolution->opts->family_prio == AF_INET) {
 		resolution->query_type = DNS_RTYPE_A;
 	} else {
 		resolution->query_type = DNS_RTYPE_AAAA;