ssh/tailssh: don't swallow process exit code in be-child

Thanks to @nshalman and @Soypete for debugging!

Updates #6054

Change-Id: I74550cc31f8a257b37351b8152634c768e1e0a8a
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