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/version_internal_test.go

@@ -25,3 +25,38 @@ func TestIsValidLongWithTwoRepos(t *testing.T) {
 		}
 	}
 }
+
+func TestPrepExeNameForCmp(t *testing.T) {
+	cases := []struct {
+		exe  string
+		want string
+	}{
+		{
+			"tailscale-ipn.exe",
+			"tailscale-ipn",
+		},
+		{
+			"tailscale-gui-amd64.exe",
+			"tailscale-gui",
+		},
+		{
+			"tailscale-gui-amd64",
+			"tailscale-gui",
+		},
+		{
+			"tailscale-ipn",
+			"tailscale-ipn",
+		},
+		{
+			"TaIlScAlE-iPn.ExE",
+			"tailscale-ipn",
+		},
+	}
+
+	for _, c := range cases {
+		got := prepExeNameForCmp(c.exe, "amd64")
+		if got != c.want {
+			t.Errorf("prepExeNameForCmp(%q) = %q; want %q", c.exe, got, c.want)
+		}
+	}
+}