tor sets `TOR_PT_EXIT_ON_STDIN_CLOSE=1` only for server transports, not client transports
At comment:2:ticket:24642, we were surprised to find that server transports get
TOR_PT_EXIT_ON_STDIN_CLOSE=1 set, and client transports do not.
We expected that it would be set for both. Client PTs have code to deal with
TOR_PT_EXIT_ON_STDIN_CLOSE, which it turns out has been going unused.