VAX: Simplified idle detection which allows more operating systems to properly detect when they are idling.

Removed the recently added SET CPU IDLE=SYSV since any SET CPU IDLE command will work for SysV.  Existing configurations probably did a SET CPU IDLE=32V which works fine now.

Changed things based on the realization that ANY branch instruction which tests memory and then branches to itself is an idle loop if the branch is taken.  This is without regard to address space, access mode, If interrupts are disabled, then it is a hung system and the simulation should halt.