cmd/tailscale/cli: prefix all --help usages with "tailscale ...", some tidying

Also capitalises the start of all ShortHelp, allows subcommands to be hidden
with a "HIDDEN: " prefix in their ShortHelp, and adds a TS_DUMP_HELP envknob
to look at all --help messages together.

Fixes #11664

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

cmd/tailscale/cli/update.go

@@ -19,7 +19,7 @@ import (
 
 var updateCmd = &ffcli.Command{
 	Name:       "update",
-	ShortUsage: "update",
+	ShortUsage: "tailscale update",
 	ShortHelp:  "Update Tailscale to the latest/different version",
 	Exec:       runUpdate,
 	FlagSet: (func() *flag.FlagSet {