The stdio listener wraps os.Stdin/os.Stdout as a net.Listener that accepts
exactly one connection. It enables GOST to be used as an SSH ProxyCommand,
where the parent process (SSH) pipes the transport byte stream through the
child process's standard I/O.
Usage: gost -L stdio://example.com:22 -F http://proxy:8080
The URL host:port is treated as the forwarding target and converted to a
forward node in buildServiceConfig. The standard forward handler then
routes traffic through the configured chain to the destination.
Fixesgo-gost/gost#433