cmd/tailscale: add shell tab-completion

The approach is lifted from cobra: `tailscale completion bash` emits a bash
script for configuring the shell's autocomplete:

    . <( tailscale completion bash )

so that typing:

    tailscale st<TAB>

invokes:

    tailscale completion __complete -- st

RELNOTE=tailscale CLI now supports shell tab-completion

Fixes #3793

Signed-off-by: Paul Scott <paul@tailscale.com>

cmd/tailscale/cli/configure-synology.go

@@ -24,9 +24,9 @@ import (
 var configureHostCmd = &ffcli.Command{
 	Name:       "configure-host",
 	Exec:       runConfigureSynology,
-	ShortUsage: "tailscale configure-host",
+	ShortUsage: "tailscale configure-host\n" + synologyConfigureCmd.ShortUsage,
 	ShortHelp:  synologyConfigureCmd.ShortHelp,
-	LongHelp:   synologyConfigureCmd.LongHelp,
+	LongHelp:   hidden + synologyConfigureCmd.LongHelp,
 	FlagSet: (func() *flag.FlagSet {
 		fs := newFlagSet("configure-host")
 		return fs