Attempt to address multiple redundant SSH related error/log messages from

the output_thread which may occur after an SSH-related error in the input
thread, e.g.:
Sun Mar 25 2018 01:45 pm bbs.synchro.net
Node 6 SSH ERROR 'Bad argument, parameter 2' (-2) setting channel

Sun Mar 25 2018 01:45 pm bbs.synchro.net
Node 6 SSH ERROR 'Bad argument, parameter 2' (-2) setting channel

Sun Mar 25 2018 01:45 pm bbs.synchro.net
Node 6 SSH ERROR 'Bad argument, parameter 2' (-2) setting channel

Sun Mar 25 2018 01:45 pm bbs.synchro.net
Node 6 SSH ERROR 'Bad argument, parameter 2' (-2) setting channel

Sun Mar 25 2018 01:45 pm bbs.synchro.net
Node 6 SSH ERROR 'Bad argument, parameter 2' (-2) setting channel

Sun Mar 25 2018 01:45 pm bbs.synchro.net
Node 6 SSH ERROR 'Bad argument, parameter 2' (-2) setting channel

... by introducing a new sbbs_t member variable: terminate_output_thread
which is only set to true when the input_thread is terminating.

If there is any possibility that the input_thread could startup and get
an SSH error before the output_thread even starts, then we will want to
move the terminate_output_thread=false initialization to the sbbs_t
constructor.