When the passthru_thead is running, the protocol (%p) command-line specifier

will now expand to "raw" rather than "telnet/rlogin/ssh". This disables IAC
handling in sexpots, for example, which isn't needed or desired when using
the passthru socket.

Also, put some conditional checks for passthru_thread_running in external()
and if it's not (e.g. for testing purposes), then lock the input_thread_mutex
instead of changing passthru_socket_active. This is currently just a way to
easily test the differences between direct and passthru socket I/O
(dependant on whether or not the passthru_thread is running which is controlled
in main.cpp.