PDP1, PDP10, PDP11, ID16, ID32, NOVA, VAX: 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.

PDP10/pdp10_lp20.c

@@ -949,7 +949,8 @@ return SCPE_OK;
 static t_stat lp20_attach (UNIT *uptr, CONST char *cptr)
 {
 t_stat reason;
-    
+
+sim_switches |= SWMASK ('A');                           /* position to EOF */
 reason = attach_unit (uptr, cptr);                      /* attach file */
 if (reason == SCPE_OK) {
     sim_fseek (uptr->fileref, 0, SEEK_END);