nixos/ejabberd: use standalone epmd service

There can only be one epmd per machine, and ejabberd will spawn it
in its context if it is not running. This is fine if ejabberd is the
only Erlang application running on the machine, but it will cause
problems when coexisting with others. Specifically when services.epmd
is enabled at the same time, this will cause a port collision if
epmd.socket starts up after ejabberd.service. Make sure that doesn't
happen by depending on services.epmd.

nixos/modules/services/networking/ejabberd.nix

@@ -93,6 +93,8 @@ in
   config = lib.mkIf cfg.enable {
     environment.systemPackages = [ cfg.package ];
 
+    services.epmd.enable = true;
+
     users.users = lib.optionalAttrs (cfg.user == "ejabberd") {
       ejabberd = {
         group = cfg.group;
@@ -109,7 +111,11 @@ in
     systemd.services.ejabberd = {
       description = "ejabberd server";
       wantedBy = [ "multi-user.target" ];
-      after = [ "network.target" ];
+      after = [
+        "network.target"
+        "epmd.socket"
+      ];
+      wants = [ "epmd.socket" ];
       path = [
         pkgs.findutils
         pkgs.coreutils