This resolves the issue where if an external program closes the
client_socket_dup, the passthru_thread can't write to the passthru_socket,
so it terminates itself - by definition, deactivating the passthru_socket, but
it didn't clear passthru_socket_active, so the input_thread would keep trying
to write to it rather than sending the input from the user to the BBS.

Also, terminate the passthru_thread (without any special log message) when
sbbs->online is non-zero.

Also overhauled a lot of the passthru-related log messages. Passthru ain't just
for SSH anymore.