clientupdate, util/osshare, util/winutil, version: improve Windows GUI filename resolution and WinUI build awareness

On Windows arm64 we are going to need to ship two different GUI builds;
one for Win10 (GOARCH=386) and one for Win11 (GOARCH=amd64, tags +=
winui). Due to quirks in MSI packaging, they cannot both share the
same filename. This requires some fixes in places where we have
hardcoded "tailscale-ipn" as the GUI filename.

We also do some cleanup in clientupdate to ensure that autoupdates
will continue to work correctly with the temporary "-winui" package
variant.

Fixes #17480
Updates https://github.com/tailscale/corp/issues/29940

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

version/exename.go

@@ -0,0 +1,25 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package version
+
+import (
+	"path/filepath"
+	"strings"
+)
+
+// prepExeNameForCmp strips any extension and arch suffix from exe, and
+// lowercases it.
+func prepExeNameForCmp(exe, arch string) string {
+	baseNoExt := strings.ToLower(strings.TrimSuffix(filepath.Base(exe), filepath.Ext(exe)))
+	archSuffix := "-" + arch
+	return strings.TrimSuffix(baseNoExt, archSuffix)
+}
+
+func checkPreppedExeNameForGUI(preppedExeName string) bool {
+	return preppedExeName == "tailscale-ipn" || preppedExeName == "tailscale-gui"
+}
+
+func isGUIExeName(exe, arch string) bool {
+	return checkPreppedExeNameForGUI(prepExeNameForCmp(exe, arch))
+}