SDS: Add assembly/disassembly support for common SYSPOPs.

Add new instruction class -- I_V_SPP for SYSPOPs and definitions for the standard ones provided by the time sharing system. Add instruction name annotation to the opc_val array.
SDS: Clean up CPU modes and record CPU mode in instruction history record

The three CPU modes -- normal, monitor and user were represented by two binary flags (nml_mode and usr_mode). The code is simpler and easier to read if the three modes are represented by one three-valued variable, cpu_mode. In addition, record the CPU mode in the instruction history record, and add the mode to the history display.

In addition, add an optional flag to the SET CPU HISTORY=n command to *not* record instructions executed in a particular CPU mode.  Flags are -n, -m and -u for normal, monitor and user mode respectively.  For example, SET -m CPU HISTORY=n will only record instructions executed in normal or user mode, and will not record instructions executed in monitor mode. This change aids user-mode program debugging by not filling the history with monitor-mode service functions and interrupt activity.
SDS: Documentation change for new history and SYSPOP features.
Merge git://github.com/simh/simh