pty: send EOF to monitor on EIO

Problem: The pty monitor callback is not called when the pty "follower"
has closed, i.e. all open handles using the pty have closed. However,
this would be extremely useful information to share with the monitor.

When an EIO error is receieved on the pty, send an EOF message to the
monitor (len == 0) to indicate there are no more pty users.

Author: grondo

src/common/libterminus/pty.c

@@ -425,6 +425,12 @@ void pty_client_send_data (struct flux_pty *pty, void *data, int len)
     }
 }
 
+void pty_client_monitor_send_eof (struct flux_pty *pty)
+{
+    if (pty->monitor)
+        (*pty->monitor) (pty, NULL, 0);
+}
+
 static void pty_read (flux_reactor_t *r,
                       flux_watcher_t *w,
                       int revents,
@@ -452,6 +458,7 @@ static void pty_read (flux_reactor_t *r,
             flux_watcher_stop (pty->fdw);
             pty->wait_on_close = false;
             check_pty_complete (pty);
+            pty_client_monitor_send_eof (pty);
             return;
         }
         llog_error (pty, "read: %s", strerror (errno));