XLAT should support segment override

[neldredge]

The 8086 XLAT instruction can take a segment override prefix. Currently the emulator has no way to express this.

One option that would be reasonably consistent with 8086 assembly would be xlat byte es [bx]. Of course no other addressing mode besides [bx] would be valid here. (Actually, I believe the original assembler would let you write some other addressing mode but just ignore it.)

To avoid the parser awkwardness, they used xlat for this "one operand form" and xlatb for the zero-operand form where DS is assumed.