PDP8: Open LPT, PTP devices for append

This implements the principle of "least surprise", in that users won't
normally expect to start overwriting an existing file on these devices.
Real hardware didn't behave that way.  A new (empty) file can always
be created with the -N switch on the ATTACH.

PDP8/pdp8_lp.c

@@ -177,6 +177,7 @@ t_stat lpt_attach (UNIT *uptr, CONST char *cptr)
 {
 t_stat reason;
 
+sim_switches |= SWMASK ('A');   /* Default to Append to existing file */
 reason = attach_unit (uptr, cptr);
 lpt_err = (lpt_unit.flags & UNIT_ATT) == 0;
 return reason;