INT 21,4D - Get Return Code of Sub-process

	AH = 4D

	on return:
	AH = system exit code  (indicates normal termination)
	   = 00 for normal termination
	   = 01 if terminated by ctl-break
	   = 02 if critical device error
	   = 03 if terminated by INT 21,31
	AL = child exit code

	- retrieve child process and system exit codes
	- this function can only be used to retrieve the exit code once
	  multiple attempts to read exit codes will cause errors