wgengine/netstack: add an SSH server experiment

Disabled by default.

To use, run tailscaled with:

    TS_SSH_ALLOW_LOGIN=you@bar.com

And enable with:

    $ TAILSCALE_USE_WIP_CODE=true tailscale up --ssh=true

Then ssh [any-user]@[your-tailscale-ip] for a root bash shell.
(both the "root" and "bash" part are temporary)

Updates #3802

Change-Id: I268f8c3c95c8eed5f3231d712a5dc89615a406f0
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/prefs_clone.go

@@ -40,6 +40,7 @@ var _PrefsCloneNeedsRegeneration = Prefs(struct {
 	ExitNodeIP             netaddr.IP
 	ExitNodeAllowLANAccess bool
 	CorpDNS                bool
+	RunSSH                 bool
 	WantRunning            bool
 	LoggedOut              bool
 	ShieldsUp              bool