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/prop.go

@@ -159,7 +159,9 @@ func IsWindowsGUI() bool {
 		if err != nil {
 			return false
 		}
-		return strings.EqualFold(exe, "tailscale-ipn.exe") || strings.EqualFold(exe, "tailscale-ipn")
+		// It is okay to use GOARCH here because we're checking whether our
+		// _own_ process is the GUI.
+		return isGUIExeName(exe, runtime.GOARCH)
 	})
 }