nixos: Add networking.fqdnOrHostName option, readOnly

nixos/modules/tasks/network-interfaces.nix

@@ -479,6 +479,22 @@ in
       '';
     };
 
+    networking.fqdnOrHostName = mkOption {
+      readOnly = true;
+      type = types.str;
+      default = if cfg.domain == null then cfg.hostName else cfg.fqdn;
+      defaultText = literalExpression ''
+        if cfg.domain == null then cfg.hostName else cfg.fqdn
+      '';
+      description = lib.mdDoc ''
+        Either the fully qualified domain name (FQDN), or just the host name if
+        it does not exists.
+
+        This is a convenience option for modules to read instead of `fqdn` when
+        a mere `hostName` is also an acceptable value.
+      '';
+    };
+
     networking.hostId = mkOption {
       default = null;
       example = "4e98920d";