hostinfo: use the sentinel value set by the MSI installer to detect MSI package type

The MSI installer sets a special sentinel value that we can use to detect it.

I also removed the code that bails out when the installation path is not
`Program Files`, as both the NSIS and MSI installers permit the user to install
to a different path.

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

cmd/tailscale/depaware.txt

@@ -81,6 +81,8 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
    W    tailscale.com/util/endian                                    from tailscale.com/net/netns
         tailscale.com/util/groupmember                               from tailscale.com/cmd/tailscale/cli
         tailscale.com/util/lineread                                  from tailscale.com/net/interfaces+
+   W    tailscale.com/util/winutil                                   from tailscale.com/hostinfo
+   W 💣 tailscale.com/util/winutil/vss                               from tailscale.com/util/winutil
         tailscale.com/version                                        from tailscale.com/cmd/tailscale/cli+
         tailscale.com/version/distro                                 from tailscale.com/cmd/tailscale/cli+
         tailscale.com/wgengine/filter                                from tailscale.com/types/netmap

hostinfo/hostinfo_windows.go

@@ -8,11 +8,11 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
-	"strings"
 	"sync/atomic"
 
 	"golang.org/x/sys/windows"
 	"golang.org/x/sys/windows/registry"
+	"tailscale.com/util/winutil"
 )
 
 func init() {
@@ -65,32 +65,20 @@ func packageTypeWindows() string {
 	if _, err := os.Stat(`C:\ProgramData\chocolatey\lib\tailscale`); err == nil {
 		return "choco"
 	}
+	if msiSentinel := winutil.GetRegInteger("MSI", 0); msiSentinel == 1 {
+		return "msi"
+	}
 	exe, err := os.Executable()
 	if err != nil {
 		return ""
 	}
 	dir := filepath.Dir(exe)
-	if !strings.Contains(dir, "Program Files") {
-		// Atypical. Not worth trying to detect. Likely open
-		// source tailscaled or a developer running by hand.
-		return ""
-	}
 	nsisUninstaller := filepath.Join(dir, "Uninstall-Tailscale.exe")
 	_, err = os.Stat(nsisUninstaller)
 	if err == nil {
 		return "nsis"
 	}
-	if os.IsNotExist(err) {
-		_, cliErr := os.Stat(filepath.Join(dir, "tailscale.exe"))
-		_, daemonErr := os.Stat(filepath.Join(dir, "tailscaled.exe"))
-		if cliErr == nil && daemonErr == nil {
-			// Almost certainly MSI.
-			// We have tailscaled.exe and tailscale.exe
-			// next to each other in Program Files, but no
-			// uninstaller.
-			// TODO(bradfitz,dblohm7): tighter heuristic?
-			return "msi"
-		}
-	}
+	// Atypical. Not worth trying to detect. Likely open
+	// source tailscaled or a developer running by hand.
 	return ""
 }