all: Remove lib/util package (#9049)

Grab-bag packages are nasty, this cleans it up a little by splitting it
into topical packages sempahore, netutil, stringutil, structutil.

lib/netutil/netutil.go

@@ -0,0 +1,18 @@
+// Copyright (C) 2023 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package netutil
+
+import "net/url"
+
+// Address constructs a URL from the given network and hostname.
+func AddressURL(network, host string) string {
+	u := url.URL{
+		Scheme: network,
+		Host:   host,
+	}
+	return u.String()
+}