TAPES: Make sure that all activities related to TPC formatted tapes are read only.

- A -R flag is forced (attach READ ONLY) when TPC formatted tapes are attached.  This should avoid updating file modified timestamps by any simh simulator.
- All write operations to TPC formatted tapes will fail with a write protected status.
SCP: Fix SET VERIFY which didn't work while processing command procedures
PDP11, VAX: Better support for various positioning operations which different OS drivers use.  Fix to properly record/tolerate skip n records reaching a tape mark. Fix to #139