ssh/tailssh: start login shell, fix arg passing, width/height mismatch

Updates #3802

Change-Id: I137d7a79195ee86d5dd7c8999f2797fc3cb57cec
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