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>

cmd/tailscale/depaware.txt

@@ -115,7 +115,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         tailscale.com/util/quarantine                                from tailscale.com/cmd/tailscale/cli
         tailscale.com/util/singleflight                              from tailscale.com/net/dnscache
         tailscale.com/util/strs                                      from tailscale.com/hostinfo+
-   W 💣 tailscale.com/util/winutil                                   from tailscale.com/hostinfo+
+     💣 tailscale.com/util/winutil                                   from tailscale.com/hostinfo+
         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