networkmanager: Expand dns description, integrate with other services (#41898)

Rather than special-casing the dns options in networkmanager.nix, use
the module system to let unbound and systemd-resolved contribute to
the newtorkmanager config.

nixos/modules/services/networking/unbound.nix

@@ -131,6 +131,9 @@ in
       };
     };
 
+    # If networkmanager is enabled, ask it to interface with unbound.
+    networking.networkmanager.dns = "unbound";
+
   };
 
 }