Use the right default user.

Index: util/startup.lua
--- util/startup.lua.orig
+++ util/startup.lua
@@ -584,7 +584,7 @@ function startup.switch_user()
 		local arg_root = prosody.opts.root;
 		if prosody.current_uid == 0 and config.get("*", "run_as_root") ~= true and not arg_root then
 			-- We haz root!
-			local desired_user = config.get("*", "prosody_user") or "prosody";
+			local desired_user = config.get("*", "prosody_user") or "_prosody";
 			local desired_group = config.get("*", "prosody_group") or desired_user;
 			local ok, err = pposix.setgid(desired_group);
 			if ok then
