flux-exec: do not operate on failed subprocesses

[chu11]

Problem: If a subprocess fails for some reason in flux-exec, it still remains in the global subprocesses list. That means that operations on the whole list, such as killing all subprocesses or sending stdin to subprocesses, will continue to operate on those failed subprocesses.

Solution: When a subprocess fails, it should be removed from the global list.

[chu11]

nevermind ... we check that the process is still running at all appropriate locations e.g.

        while (p) {                                                                                                                         
            if (flux_subprocess_state (p) == FLUX_SUBPROCESS_INIT                                                                           
                || flux_subprocess_state (p) == FLUX_SUBPROCESS_RUNNING) {                                                                  
                if (flux_subprocess_write (p, "stdin", ptr, lenp) < 0)                                                                      
                    log_err_exit ("flux_subprocess_write");                                                                                 
            }                                                                                                                               
            p = zlist_next (subprocesses);                                                                                                  
        }