SDS: Support simulator's new NEXT command to step over subroutine calls

Step over BRM, POP or SYSPOP by planting temporary (dynamic) breakpoints at P+1 and P+2.
Merge pull request #132 from markemmer/master

SDS: Support simulator's new NEXT command to step over subroutine calls
SDS: Add -f switch to Next command to force step-over mode

Add -f switch ("force") to the Next command to set the temporary breakpoints regardless of instruction type.  This is useful at
the bottom of loops or to avoid going off into unrelated code should an interrupt or memory paging trap occur.
Merge git://github.com/simh/simh
SCP: Added a repeat count for the NEXT command
PDP11: Added support for the NEXT command to step over subroutine calls.
PDP10: Added support for the NEXT command to step over subroutine calls.
SCP: Added the rest of the NEXT command repeat count support.
SCP: Fix errant display of NEXT warning on first STEP command.  From Peter Schorn

Better display in SHOW VERSION of full compiler name on OSX when using the clang LLVM compilers
Change stray non-ascii quote characters in comments.
VAX, PDP11, PDP10: Added a explanation of the capabilities and limitations of the NEXT command which is output the first time a NEXT command is entered.

Also added CHMK, CHME, CHMS and CHMU to the set of instructions which are considered subroutine call initiators in the VAX simulator.
SDS: Add more complex (but optional) Next behavior

Allow -f switch with Next to move forward in code.
Allow -a switch with Next for "atomic" behavior that excludes interrupts for all instructions.

Update sds_doc.doc to describe the new behavior.
Added debug support for the MHDSK device, support for the NEXT command and a warning if 8086 symbolic input is attempted.