The lack of PTs having a reliable way to detect if tor is dead reared it's ugly head again in the form of legacy/trac#15434. There aren't that many good ways to detect if a parent has died, beyond checking if stdin is still open (legacy/trac#10047 says stdout, however stdin is better), but tor explicitly closes the child process's stdin. Since a variant of this has been working well, I propose that we make it official like thus: * Add a new enviornment variable of the form `TOR_PT_EXIT_ON_STDIN_CLOSE=1`, indicating that Tor will not close the stdin. * PTs SHOULD terminate as if they have received a SIGTERM, on stdin being closed if the environment variable is set. * All new tor versions will naturally set said env. var, and setup the pt plugin stdin correctly. This does mean that Bridges will consume one extra file descriptor per `ServerTransportPlugin` line in the torrc, but I view this as acceptable given the fact that this will bring some much needed sanity to our cleanup process.