PDP1, PDP8, PDP18B, PDP11: Fixed dt_seterr to clear successor states

For the PDP11, this includes deferring the setting of DONE on an error, allowing RSTS V4 to work correctly.

There was a minor (possibly only theoretical) bug in dt_seterr as well.
dt_seterr is supposed to stop the drive. If the drive is accelerating, at
speed, or stopped, it works fine. But if the drive is decelerating, it did
not clear out the "successor" states. In theory, the drive could be in the
middle of reversing - that is, there are successor states of accelerating
(in reverse) and at speed. So the successor states need to be cleared,
or the drive won't actually stop.

The fifth DECtape controller (the PDP8's TD8E) is much simpler and
doesn't have the problem.

PDP18B/pdp18b_dt.c

@@ -27,6 +27,7 @@
                 (PDP-9) TC02/TU55 DECtape
                 (PDP-15) TC15/TU56 DECtape
 
+   15-Mar-17    RMS     Fixed dt_seterr to clear successor states
    09-Mar-17    RMS     Fixed dt_seterr to handle nx unit select (COVERITY)
    10-Mar-16    RMS     Added 3-cycle databreak set/show entries
    07-Mar-16    RMS     Revised for dynamically allocated memory
@@ -1283,6 +1284,7 @@ if (uptr != NULL) {                                     /* valid select? */
         sim_activate (uptr, dt_dctime);                 /* sched decel */
         DTS_SETSTA (DTS_DECF | (mot & DTS_DIR), 0);     /* state = decel */
         }
+    else DTS_SETSTA (mot, 0);                           /* clear 2nd, 3rd */
     }
 DT_UPDINT;
 return;