MINOR: resolvers: rework prototype suffixes to split resolving and dns.

A lot of prototypes in dns.h are specific to resolvers and must
be renamed to split resolving and DNS layers.
diff --git a/src/tools.c b/src/tools.c
index 003fb52..2f7b5f1 100644
--- a/src/tools.c
+++ b/src/tools.c
@@ -737,7 +737,7 @@
 	if (!resolve)
 		return NULL;
 
-	if (!dns_hostname_validation(str, NULL))
+	if (!resolv_hostname_validation(str, NULL))
 		return NULL;
 
 #ifdef USE_GETADDRINFO