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.

PDP8/pdp8_dt.c

@@ -1,6 +1,6 @@
 /* pdp8_dt.c: PDP-8 DECtape simulator
 
-   Copyright (c) 1993-2013, Robert M Supnik
+   Copyright (c) 1993-2017, Robert M Supnik
 
    Permission is hereby granted, free of charge, to any person obtaining a
    copy of this software and associated documentation files (the "Software"),
@@ -25,6 +25,7 @@
 
    dt           TC08/TU56 DECtape
 
+   15-Mar-17    RMS     Fixed dt_seterr to clear successor states
    17-Sep-13    RMS     Changed to use central set_bootpc routine
    23-Jun-06    RMS     Fixed switch conflict in ATTACH
    07-Jan-06    RMS     Fixed unaligned register access bug (Doug Carman)
@@ -1073,6 +1074,7 @@ if (mot >= DTS_ACCF) {                                  /* ~stopped or stopping?
     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;
 }