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()
+}

lib/netutil/netutil_test.go

@@ -0,0 +1,28 @@
+// 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 "testing"
+
+func TestAddress(t *testing.T) {
+	tests := []struct {
+		network string
+		host    string
+		result  string
+	}{
+		{"tcp", "google.com", "tcp://google.com"},
+		{"foo", "google", "foo://google"},
+		{"123", "456", "123://456"},
+	}
+
+	for _, test := range tests {
+		result := AddressURL(test.network, test.host)
+		if result != test.result {
+			t.Errorf("%s != %s", result, test.result)
+		}
+	}
+}