cmd/tailscale/cli: add "help" alias for --help

Fixes #14053

Change-Id: I0a13e11af089f02b0656fea0d316543c67591fb5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f