Expects: AX    4b00H
         DS:DX address of an ASCIIZ- filespec of the program to execute
         ES:BX address of an ExecParamRec
         ------------------------------------------------------------------
Returns: AX    error code if CF is set to CY
         else  All registers are destroyed, including SS and SP
         ------------------------------------------------------------------
   Info: Provides a means for a program (the parent) to execute another
         program (the child).  When the child exits, control will return
         to the parent.  It prepares a PSP for the child, loads its code,
         performs any needed segment fixup, est the registers as expected,
         then begins executing the child program.

   DS:DX points to an ASCIIZ string in the form...
            d:\path\filename.ext 0 
         If the drive or path is omitted, defaults are assumed.

   ES:BX points to a block of memory prepared as an ExecParamRec that has
         been filled with the information needed.