networking: enable "multi on" in resolver settings

this allows to return ipv4/ipv6 addresses for the same host in /etc/hosts.
fixes #19148

nixos/modules/config/networking.nix

@@ -29,6 +29,19 @@ in
       '';
     };
 
+    networking.hostConf = lib.mkOption {
+      type = types.lines;
+      default = "multi on";
+      example = ''
+        multi on
+        reorder on
+        trim lan
+      '';
+      description = ''
+        The contents of <filename>/etc/host.conf</filename>. See also <citerefentry><refentrytitle>host.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
+      '';
+    };
+
     networking.dnsSingleRequest = lib.mkOption {
       type = types.bool;
       default = false;
@@ -171,6 +184,9 @@ in
             ${cfg.extraHosts}
           '';
 
+        # /etc/host.conf: resolver configuration file
+        "host.conf".text = cfg.hostConf;
+
         # /etc/resolvconf.conf: Configuration for openresolv.
         "resolvconf.conf".text =
             ''