ssh/tailssh,ipn/ipnlocal: terminate any active sessions on up --ssh=false

Currently the ssh session isn't terminated cleanly, instead the packets
are just are no longer routed to the in-proc SSH server. This makes it
so that clients get a disconnection when the `RunSSH` pref changes to
`false`.

Updates #3802

Signed-off-by: Maisem Ali <maisem@tailscale.com>

ipn/ipnlocal/local.go

@@ -1912,6 +1912,10 @@ func (b *LocalBackend) setPrefsLockedOnEntry(caller string, newp *ipn.Prefs) {
 		b.authReconfig()
 	}
 
+	if oldp.RunSSH && !newp.RunSSH && b.sshServer != nil {
+		go b.sshServer.OnPolicyChange()
+	}
+
 	b.send(ipn.Notify{Prefs: newp})
 }