logpolicy, ipn/ipnserver: connect to logtail via tailscaled when needed

This is for use by the Windows GUI client to log via when an
exit node is in use, so the logs don't go out via the exit node and
instead go directly, like tailscaled's. The dialer tried to do that
in the unprivileged GUI by binding to a specific interface, but the
"Internet Kill Switch" installed by tailscaled for exit nodes
precludes that from working and instead the GUI fails to dial out.
So, go through tailscaled (with a CONNECT request) instead.

Fixes tailscale/corp#3169

Change-Id: I17a8efdc1d4b8fed53a29d1c19995592b651b215
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

version/prop.go

@@ -62,3 +62,13 @@ func IsMacSysExt() bool {
 	isMacSysExt.Store(v)
 	return v
 }
+
+// IsWindowsGUI reports whether the current process is the Windows GUI.
+func IsWindowsGUI() bool {
+	if runtime.GOOS != "windows" {
+		return false
+	}
+	exe, _ := os.Executable()
+	exe = filepath.Base(exe)
+	return strings.EqualFold(exe, "tailscale-ipn.exe") || strings.EqualFold(exe, "tailscale-ipn")
+}