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.