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/down.go

@@ -14,7 +14,7 @@ import (
 
 var downCmd = &ffcli.Command{
 	Name:       "down",
-	ShortUsage: "down",
+	ShortUsage: "tailscale down",
 	ShortHelp:  "Disconnect from Tailscale",
 
 	Exec:    runDown,