Release High Memory Area (Function 02h)

        ARGS:   AH = 02h

        RETS:   AX = 0001h if the HMA is successfully released,
                     0000h otherwise

        ERRS:   BL = 80h if the function is not implemented
                BL = 81h if a VDISK device is detected
                BL = 90h if the HMA does not exist
                BL = 93h if the HMA was not allocated

This function releases the high memory area and allows other programs to
use it. Programs which allocate the HMA must release it before exiting.
When the HMA has been released, any code or data stored in it becomes
invalid and should not be accessed.