resolved: do not start LLMNR or mDNS stack when no network enables them

When no network enables LLMNR or mDNS, it is not necessary to create
LLMNR or mDNS related sockets. So, let's create them only when
LLMNR- or mDNS-enabled network becomes active or at least one network
enables `LLMNR=` or `MulticastDNS=` options.

src/resolve/resolved-link.c

@@ -28,6 +28,8 @@
 #include "mkdir.h"
 #include "parse-util.h"
 #include "resolved-link.h"
+#include "resolved-llmnr.h"
+#include "resolved-mdns.h"
 #include "string-util.h"
 #include "strv.h"
 
@@ -523,10 +525,25 @@ static void link_read_settings(Link *l) {
 }
 
 int link_update(Link *l) {
+        int r;
+
         assert(l);
 
         link_read_settings(l);
         link_load_user(l);
+
+        if (l->llmnr_support != RESOLVE_SUPPORT_NO) {
+                r = manager_llmnr_start(l->manager);
+                if (r < 0)
+                        return r;
+        }
+
+        if (l->mdns_support != RESOLVE_SUPPORT_NO) {
+                r = manager_mdns_start(l->manager);
+                if (r < 0)
+                        return r;
+        }
+
         link_allocate_scopes(l);
         link_add_rrs(l, false);
 

src/resolve/resolved-manager.c

@@ -612,14 +612,6 @@ int manager_start(Manager *m) {
         if (r < 0)
                 return r;
 
-        r = manager_llmnr_start(m);
-        if (r < 0)
-                return r;
-
-        r = manager_mdns_start(m);
-        if (r < 0)
-                return r;
-
         return 0;
 }