SDS: Refine Next command for subroutine branch

1. Treat the SBRM SysPOP like the BRM opcode for calling a subroutine.
2. If the -a (atomic) switch is present with a BRM or SBRM instruction, place a breakpoint at EA+1 rather than EA, since the return link is placed at EA (BRM) or at *EA (SBRM) with subroutine execution commencing at EA+1.
Merge git://github.com/simh/simh
SCP: Fix reporting of console's buffering status.  Fix parsing of MUX settings which were simply interpreted by matching a single token. Reported by Mark Bensen.
SCP: Fix buffered console implementation to correctly retain buffering attributes across independent SET CONSOLE commands
PDP10, PDP11, VAX: Fix DMC/DMR master clear when no units are attached to properly be able to act on register writes.
PDP10, PDP11, VAX: Avoid returning "Line Unit" test failed after DMC/DMR master clear if a line is not attached.  Let the attached state (if it needs to be determined) be reflected by sensing modem status bits.
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
PDP11, VAX: Added ability to set the size of RAUSER devices to be n binary MB (1024*1024) with the -B switch.  Added SHOW -B to display drive capacities in binary MB using the SHOW -B switch.
OSX build: Added more flexible toolchain support for more recent OS X and xCode versions.
OSX build: Revise to fix prior change.
VAX730: Fix 3MB memory setting.  Found by Brad Parker.
PDP11, VAX780, VAX8600, VAX750, VAX730: Fix second DEUNA/DELUA device which would crash the simulator when detaching.  Problem reported by Jordi Guillaumes Pons.
PDP10, PDP11, VAX: Revert effort to Avoid returning "Line Unit" test failed after DMC/DMR master clear if a line is not attached.  To be addressed more thoroughly later.
AltairZ80: Added Motorola 68000 CPU support for CP/M-68K, support for Altair mini-disk format, added descriptions for all device registers