net/socks5/tssocks, wgengine: permit SOCKS through subnet routers/exit nodes

Fixes #1970

Change-Id: Ibef45e8796e1d9625716d72539c96d1dbf7b1f76
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