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