uslash including fixed eforth variant

Uslash.asm includes now both variants: First the Talbot Systems one, which is from FIG and erroneous! The second one I have created and added into eForth replacing the high level implementation with influence from various sources (several other Forth implementations, 6502, 6800, 68K, ...). Currently the eForth variant is "active" one.
Some test values and their outcome are included to cover all possible (even special) cases.
BTW: I left the clc/sec syntax omitted for sake of compatibility. ;)

Would you merge back the uslash branch into the master?

Johann