cmd/tailscale/cli: make set without args print usage

Make "tailscale set" by itself be equivalent to "tailscale set -h"
rather than just say "you did it wrong" and make people do another -h
step.

Change-Id: Iad2b2ddb2595c0121d2536de5b78648f3eded3e3
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/set.go

@@ -110,8 +110,7 @@ func runSet(ctx context.Context, args []string) (retErr error) {
 	})
 
 	if maskedPrefs.IsEmpty() {
-		println("no flags specified")
-		return nil
+		return flag.ErrHelp
 	}
 
 	if maskedPrefs.RunSSHSet {