nixos ddclient: support multiple domains and run via systemd timer

a) Some providers can update multiple domains - support that.

b) Make "zone" and "script" configurable. Some providers require these.

c) Instead of leaving the ddclient daemon running all the time, use a systemd
timer to kick it off.

d) Don't use a predefined user - run everything via DynamicUser

e) Add documentation

nixos/modules/misc/ids.nix

@@ -56,7 +56,7 @@
       #dialout = 27; # unused
       polkituser = 28;
       #utmp = 29; # unused
-      ddclient = 30;
+      # ddclient = 30; # converted to DynamicUser = true
       davfs2 = 31;
       #disnix = 33; # unused
       osgi = 34;
@@ -343,7 +343,7 @@
       dialout = 27;
       #polkituser = 28; # currently unused, polkitd doesn't need a group
       utmp = 29;
-      ddclient = 30;
+      # ddclient = 30; # converted to DynamicUser = true
       davfs2 = 31;
       disnix = 33;
       osgi = 34;