Return the length of an ASCIIZ string (excluding the terminator).

If a segment register is not provided then ES is assumed.

Unless the location is a code label, then CS is assumed.

Examples:

    StrLen cx, es:di        ; get length of string at ES:DI
    StrLen cx, bx           ; get length of string at ES:BX
    StrLen bx, AUTHOR       ; get length of string at CS:AUTHOR