Add DEC ASCII 026 and 029 translation tables.

These are used by newer DEC OSs, including TOPS-10 and TOPS-20.

The original tables (which map lower case to upper, and error on control characters) remain.

The default for the CD20 is the full ASCII 029 table, as the systems it shipped on used this.  The default for other models remains the subset 029.  The default (but not the active table) will change if the unit type is changed.