cmd/tailscale: add start of "tailscale update" command

Goal: one way for users to update Tailscale, downgrade, switch tracks,
regardless of platform (Windows, most Linux distros, macOS, Synology).

This is a start.

Updates #755, etc

Change-Id: I23466da1ba41b45f0029ca79a17f5796c2eedd92
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/winutil/winutil_notwindows.go

@@ -27,3 +27,5 @@ func isSIDValidPrincipal(uid string) bool { return false }
 func lookupPseudoUser(uid string) (*user.User, error) {
 	return nil, fmt.Errorf("unimplemented on %v", runtime.GOOS)
 }
+
+func IsCurrentProcessElevated() bool { return false }