ssh/tailssh: add Plan 9 support for Tailscale SSH

Updates #5794

Change-Id: I7b05cd29ec02085cb503bbcd0beb61bf455002ac
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f