cmd/tailscale/cli: use double hypens, make default usage func more clear

Mash up some code from ffcli and std's flag package to make a default
usage func that's super explicit for those not familiar with the Go
style flags. Only show double hyphens in usage text (but still accept both),
and show default values, and only show the proper usage of boolean flags.

Fixes #1353
Fixes #1529

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

cmd/tailscale/cli/status.go

@@ -27,7 +27,7 @@ import (
 
 var statusCmd = &ffcli.Command{
 	Name:       "status",
-	ShortUsage: "status [-active] [-web] [-json]",
+	ShortUsage: "status [--active] [--web] [--json]",
 	ShortHelp:  "Show state of tailscaled and its connections",
 	Exec:       runStatus,
 	FlagSet: (func() *flag.FlagSet {